The UN's World Health Organization has raised the alert over swine flu to level five - indicating human-to-human transmission in at least two countries.
It is a "strong signal that a pandemic is imminent", the WHO says.
In Mexico, at the epicentre of the outbreak, people have been urged to stay at home over the next five days.
There are numerous cases elsewhere - the highest number outside Mexico is the US - and Europeans have been told it is certain there will be deaths.
Several countries have restricted travel to Mexico and many tour operators have cancelled holidays.
Other countries are resisting calls to implement travel bans or close borders, on the grounds - backed by the WHO - that there is little evidence of their efficacy.
In the latest developments:
'Urgent action'
Announcing the latest alert level after an emergency WHO meeting in Geneva, Director General Margaret Chan urged all countries to activate their pandemic plans, including heightened surveillance and infection-control measures.
CONFIRMED CASES
She said action should be undertaken with "increased urgency".
She added: "It really is the whole of humanity that is under threat in a pandemic."
But she also said the world was "better prepared for an influenza pandemic than at any time in history".
Ms Chan stressed on Wednesday that there was no danger from eating properly cooked pork.
She advised hygiene measures such as hand-washing to prevent infection and said it was important "to maintain a level of calm".
Meanwhile in Mexico, President Felipe Calderon has announced the partial suspension of non-essential work and services from 1 to 5 May - a holiday period there.
In a TV address, he urged people to stay in with their families - saying there was "no place as safe as your own home".
He said he was "proud" of the response of Mexicans to the crisis, and assured people Mexico was well-stocked with anti-viral medicines.
Already, schools across Mexico have closed, public gatherings are restricted and archaeological sites have been placed off-limits.
From Stephen Gibbs in Mexico City, Mexico
Since the swine flu outbreak began here, the government has faced the dilemma of wanting to prevent people spreading the disease - without paralysing the economy.
With this latest directive it appears to have struck a compromise.
All "non-essential" areas of the economy are to be shut down, for five days from 1 to 5 May.
The government has not been specific about what it means by non essential - but it does say medical, food, transportation and financial sectors will function as normal.
Mexico is already being hit hard by the global economic slowdown, and the country's finance minister says swine flu could cut a further half-percent of GDP.
The search for the source of the outbreak continues, with the focus on the vicinity of a pig farm in the eastern part of the country.
The Mexican government is urging against jumping to conclusions and is suggesting the possibility remains that the virus originated outside the country.
Officials have put the number of suspected deaths from swine flu in Mexico at 168, although just eight deaths have been confirmed, with 26 infections positively tested.
In Europe, the director-general of health and consumer protection, Robert Madelin, said the continent was well prepared but nonetheless deaths from the disease were expected.
"It is not a question of whether people will die, but more a question of how many. Will it be hundreds, thousands or tens of thousands?", he said, speaking to Reuters news agency.
Movement bans?
At the meeting of health ministers on Thursday, a French proposal of issuing a continent-wide travel advisory for Mexico will be discussed - though it is unclear whether it is in the power of the EU executive to impose such a ban.
SYMPTOMS - WHAT TO DOSpain has seen the first case of a person contracting swine flu without having travelled there.
After Mexico, the US has recorded the next highest number of confirmed cases, with 91 - and the first death of swine flu outside Mexico, after a visiting Mexican child died in Texas.
President Barack Obama has urged local public-health bodies to be vigilant and said schools with confirmed cases "should consider closing". About 100 have so far done so.
There are no current plans to close the border with Mexico, Mr Obama said on Wednesday evening.
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World Health Organization
Government of Mexico
US Centers for Disease Control and Prevention
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