At least 57 people have been killed and about 300 injured in explosions at hotels in the Jordanian capital, Amman.
The Grand Hyatt, Radisson and Days Inn hotels were hit in near-simultaneous blasts. Police suspect suicide bombers.
The hotels are popular with foreigners but most of the victims were Jordanian, many of them celebrating a wedding.
Jordan, a key US ally in the Middle East, has long been regarded as a prime target for attacks by radical Islamic militants, correspondents say.
"Three terrorist operations targeted the Radisson SAS, the Grand Hyatt and the Days Inn hotels," police spokesman Bashir al-Daajeh told Jordanian news agency Petra.
Deputy Prime Minister Marwan Muasher told CNN television that two of the blasts appear to have been carried out by suicide bombers strapped with explosive, and the third by a suicide car bomb.
No details have emerged so far of the names or nationalities of the victims, but officials say most are thought to be Jordanian.
Many of the casualties are thought to have been among 250 guests at a wedding reception in the Radisson hotel.
"We thought it was fireworks for the wedding, but I saw people falling to the ground," said Ahmed, a wedding guest who did not give his surname.
"I saw blood. There were people killed. It was ugly."
The BBC's Caroline Hawley, who was staying at the Hyatt, says the device apparently went off in a bar in the lobby.
A favourite with businessmen and Westerners, the hotel was packed at the time.
Our correspondent says windows were blown out by the blast, and she saw several badly wounded people. Many of the injured were taken to hospital in taxis and private cars.
There was very little security apparent at the hotel prior to the blast, she adds.
There was no immediate claim of responsibility, but a Jordanian official said the attacks carried the trademark of al-Qaeda or those inspired by it.
Police have set up roadblocks around hotels and embassies, and Prime Minister Adnan Badra has ordered all schools and public offices to close on Thursday.
Jordan has reportedly closed its borders
King Abdullah II said the deadly blasts were "terrorist" acts" and pledged that "justice will pursue the criminals".
A White House spokesman condemned the bombings as "a heinous act of terror".
UN chief Kofi Annan has postponed a scheduled visit to Jordan scheduled for Thursday.
The BBC's Jordan correspondent Jon Leyne says Jordanians had been expecting this for months.
Jordan is the main US ally in the Arab world, and King Abdullah has been planning a visit to the US - as well as to Israel and the West Bank, our correspondent says.
In the past few years, Amman has also become a base for Westerners who fly in and out of Iraq for work.