A security alert was triggered at airports in the US and UK, prompted by suspect packages from Yemen, the authorities said. Part of East Midlands airport was cordoned off, and planes were searched at Newark, New Jersey, and at Philadelphia. US authorities said two packages appeared to contain explosive materials and were a credible terrorist threat. Here is a moment-by-moment account of the unfolding story, including news, analysis from our correspondents, some of your emails and tweets (all times GMT).
Ms May's statement adds: "At this stage I can say that the device did contain explosive material. But it is not yet clear that it was a viable explosive device. The forensic work continues."
UK Home Secretary Theresa May says in a statement that a package originating from Yemen is being examined by UK experts. Security measures for air freight from Yemen are being reviewed.
But President Obama was probably politically wise to make it look as though he is in personal charge of a grave situation, the BBC's Mark Mardell adds.
Read his comments in full here.
The BBC's Mark Mardell says that even with the US mid-term elections round the corner, it is unlikely the alert will have much political impact.
Three UPS planes and their cargo held at Newark and Philadelphia airports for inspection have been cleared by the US Transportation Security Administration, a UPS spokeswoman tells Reuters news agency.
The official Yemeni statement also says no UPS cargo planes have taken off from Yemen, and there have been no direct or indirect flights to British or American airports.
The Yemeni government says in a statement it is co-operating with the US, British and Emirati authorities.
Daniel Diaz from Monterrey, Mexico, writes: "I'm in Dallas-Fort Worth airport and all is peachy here, with flights arriving and departing just fine."
The authorities in Yemen are astonished at reports linking the country to the packages, the AP news agency reports.
We've had a chance to parse Mr Brennan's comments to reporters earlier, and a line about the packages stands out. "They were in a form that was designed to try to carry out some type of attack," he said.
Now Homeland Security adviser John Brennan has thanked the Saudi Arabians for their help. "Their assistance, along with the hard work of the US counter-terrorism community, the United Kingdom, the UAE, and other friends and partners helped make it possible to increase our vigilance and identify the suspicious packages in Dubai and East Midlands Airport," he says.
The intelligence tip that led to the discovery of the packages came from the Saudi Arabian authorities, a US official tells the BBC Washington bureau.
The route of the suspect package was from Yemen to Cologne and then on for a scheduled stop at East Midlands.
The UK government sources say tests are still ongoing on the explosive material and there's no conclusion that it was a viable device.
UK government confirms what Barack Obama said some time ago, that the package at East Midlands airport contained explosive material, the BBC's Danny Shaw says.
UK Home Secretary Theresa May briefed US Department of Homeland Security Secretary Janet Napolitano on developments, a Home Office spokesman told Reuters.
The UK's emergency committee Cobra met today and will meet again tomorrow.
Just picking up some comments from John Reid, former UK home secretary. He urges the government to "think again" about suggestions the inconvenience of airport security measures could be eased.
For the uninitiated, PETN is pentaerythritol tetranitrate, a very powerful explosive.
The explosive material was PETN, the same used in the Christmas Day attempted attack, AP quotes US officials as saying.
Shipping firm UPS has suspended all service out of Yemen until further notice.
The US Transportation Security Administration gives final confirmation that the planes under scrutiny, at Newark Liberty and Philadelphia International airport, have been swept and cleared.
Senator Susan Collins, ranking member of the Senate Homeland Security Committee, says she has been told by the Transportation Security Administration that additional security measures will see "an immediate ground halt on air cargo coming to the United States from Yemen and the deployment of additional inspectors to Yemen".
Anon writes: "We were evacuated from work today, at first we all thought 'ohh, a bit of excitement' but as the day went on and we were not allowed back to the office and a threat arose of widening the cordon area, reality struck of how serious the situation was and it was quite worrying. I wish the world could just live in peace."
John, Wiltshire, UK, writes: "Call me a cynic but the day after the head of BA said the security checks are over the top... this happens. Expect tighter security and even more checks now."
The BBC's sources in the UK are so far unable to confirm Mr Obama's assertion that the East Midlands package contained explosive material. The sources point out that the American authorities are receiving information as soon as it comes in, so they may be ahead of what officials here are being told, the BBC's Danny Shaw says.
The BBC's Gordon Corera says we have learned in the last hour that this wasn't some dry run, or some hoax or an attempt just to scare people. "This was some kind of tip-off," our correspondent says. "They weren't discovered by chance."
The White House releases a picture of the president being briefed in the Situation Room
on its Flickr account.
It would be unusual for a "traditional dry run" for an attack to be carried out with actual explosive material, Mr Brennan says.
On its website,
the Jewish Federation of Metropolitan Chicago says it has told Chicago synagogues to evacuate and call authorities if they receive any packages from Yemen or from any organisation with the name "Yemen" in it.
Mr Brennan says the devices were like "bread boxes" in terms of size, and says investigators do not know how they were intended to be activated.
The Homeland Security adviser reiterates that other packages from Yemen and elsewhere will be examined during the investigation.
Mr Brennan says the East Midlands package was the first to be found.
Mr Brennan says he spoke to President Ali Abdallah Saleh of Yemen, discussing co-operation and says "over the past 22 months or so... there has been a steady improvement in that co-operation". But he adds: "It needs to improve more... they are courageous partners."
Mr Brennan says an "abundance of caution" was behind the decision by Canada and US to escort an Emirates flight into JFK using fighter jets.
Mr Brennan says the devices were intended "to do harm".
John Brennan, Assistant to the President and Deputy National Security Adviser for Homeland Security and Counterterrorism, says the packages have been isolated and "made inert".
White House spokesman Robert Gibbs reads out a very long list of all of the US security bodies that have been involved in tackling the threat.
Nicholas Brown from Nottingham is a plane enthusiast who spent much of the day at East Midlands airport, UK. He writes: "Today I have had to go off airport grounds to get my pictures. The staff at the airport have been very helpful but it is mayhem down there."
Mr Obama's statement brings us the biggest nugget of information we've had all day. Among the most important elements is that the packages found in Dubai and the East Midlands were addressed to two places of Jewish worship in Chicago.
President Obama says: "Counterterrorism professionals are taking this threat very seriously." He says the commitment remains to "root out violent extremism in all its forms".
Bert Bentley from Woodlyn, PA, writes: "I am employed by a major airline at the Philadelphia International airport. From reports here this could be a probe to see how authorities respond to a possible terror threat. There are two planes now being searched here."
Mr Obama confirms that both suspicious packages did apparently contain explosive material.
President Barack Obama says the US will "spare no effort" in the investigations.
Reporters are gathered for the statement by the president at the Brady Press Briefing room, awaiting Mr Obama.
The UAE civil aviation body has indicated that the suspicious package in Dubai has been taken to a lab for tests, the state news agency says.
US intelligence warned last month of a threat involving the posting of biological and chemical agents as part of an attack, the AP reports. The bulletin on 23 September was issued by the Department of Homeland Security.
Emirates airline is co-operating with the US authorities in the investigation of the package from Yemen it was carrying, it says in a statement.
Both the AP and PA news agencies are reporting that the suspicious package in Dubai was an explosive device, quoting an anonymous source, but there is no confirmation.
The Emirates flight lands at New York's JFK.
The Emirates flight is now expected to land at New York's JFK at 1944.
The Emirates flight was due to land at New York's JFK at 1923.
The BBC's Katy Watson in Dubai says Fedex is holding all packages that are coming from Yemen. She says Dubai is a major cargo and passenger transit hub in the Middle East.
The BBC's Laura Trevelyan says that President Obama has to tread carefully lest he be accused by the Republicans of playing up the scare for political purposes ahead of Tuesday's election.
The plane being escorted is Emirates flight 201, the US Federal Aviation Administration says. It was escorted through Canadian airspace by two CF-18s and once it entered US airspace two F-15s picked it up.
CBSRadioNews tweets: Law enforcement source confirms suspicious packages were toner cartridges with syringes, white powder, wiring and cell phone components.
Read CBSRadioNews's tweets
The BBC's Steve Kingstone in Washington says US officials are taking the threat very, very seriously. He says the president was notified of a potential terrorist threat on Thursday at 2230 local time, and US officials worked through the night in partnership with officials in Britain and Dubai.
The FBI says there is no known threat associated with the airliner being escorted by fighters, Reuters reports. It simply carries a package from Yemen.
Former secretary of homeland security Tom Ridge tells Fox News that the fact the president was personally informed about the threat suggests the information came from a credible source.
"There are no identifiable or specific threats to the Chicago area," says Special Agent Ross Rice, of the Chicago FBI. "Since two of the suspicious packages that were intercepted were addressed to religious institutions in Chicago, all churches, synagogues and mosques in the Chicago area should be vigilant for any unsolicited or unexpected packages, especially those originating from overseas locations."
Salon.com reporter Justin Elliot asks on Twitter: "serious question on "dry run" stuff: if you're a terrorist and can get a bunch of packages on planes, why not just make it real bombs?"
The airliner being escorted began its journey in Yemen and came via the UAE, Fox News reports.
Fighter jets are escorting a passenger flight from UAE into New York's JFK airport, CNN reports.
President Barack Obama will give a statement on the alert at about 2015 GMT.
The home secretary also notes the suspicious package from East Midlands airport is still being examined and reiterates that there is no evidence of a threat to the UK.
UK Home Secretary Theresa May says authorities are considering what steps to take about cargo coming from Yemen and says there are no direct flights from Yemen to the UK proceeding at the moment.
The BBC's Laura Trevelyan at Newark airport in the state of New Jersey says cargo planes there are kept on an isolated perimeter, which is why they are seen as vulnerable.
The Anti-Defamation League says it has issued a warning of a threat to Jewish institutions in the US, the Reuters news agency reports.
No explosives were found in two suspect packages that were being sent to the US, news agency AFP quotes the FBI as saying. The FBI does not think an attack is imminent.
There is strong suspicion that packages will have been sent by al-Qaeda in the Arabian peninsula, says BBC's Gordon Corera.
Yemeni authorities have launched an investigation into the suspicious packages.
Two packages were addressed to religious buildings in Chicago, the FBI is quoted as saying by news agency AFP.
All clear given to plane being searched at Newark airport.
Security expert Dr John Gearson tells the BBC the alert could have been sparked by a "dry run".