Updated 11.20am

Sri Lanka declared a state of emergency on Wednesday after the president fled overseas to escape anti-government protests, as thousands of people took to the streets, again furious that he had not yet stood down.

Gotabaya Rajapaksa had promised at the weekend to resign on Wednesday and clear the way for a "peaceful transition of power" after fleeing his official residence in Colombo just before tens of thousands of protesters overran it.

As president, Rajapaksa enjoys immunity from arrest, and he is believed to have wanted to go abroad before stepping down to avoid the possibility of being detained.

He, his wife and two bodyguards were the four passengers on a military aircraft that took off from Sri Lanka's main international airport, immigration sources told AFP.

Hours later, with no formal announcement he was stepping down, thousands of demonstrators mobbed the office of Prime Minister Ranil Wickremesinghe - whom Rajapaksa named as acting president during his absence - demanding both officeholders should go.

"Go home Ranil, Go home Gota," they shouted.

Police fired tear gas to hold them back from overrunning the compound and officials declared a nationwide state of emergency, the prime minister's spokesman Dinouk Colombage told AFP.

Police imposed an indefinite curfew across the Western Province, which includes Colombo, "to contain the situation", a senior police officer said.

Wickremesinghe would automatically become acting president if Rajapaksa steps down, but has himself announced his willingness to resign if consensus is reached on forming a unity government.

The prime minister's office confirmed that Rajapaksa had left the country, but said it had no schedule for any presidential resignation announcement.

The succession process could take between three days - the minimum time needed for parliament to elect an MP to serve out Rajapaksa's term, which ends in November 2024 - and a maximum of 30 days allowed under the statute. 

'The Terminator'

The departure of the 73-year-old leader once known as 'The Terminator' had been stymied for more than 24 hours in a humiliating stand-off with immigration personnel in Colombo.

He had wanted to fly to Dubai on a commercial flight, but staff at Bandaranaike International withdrew from VIP services and insisted that all passengers had to go through public counters. 

The presidential party were reluctant to go through regular channels fearing public reactions, a security official said, and as a result, missed four flights on Monday that could have taken them to the United Arab Emirates.

Clearance for a military flight to land in the closest neighbour India was not immediately secured, a security official said, and at one point on Tuesday the group headed to a naval base with a view to fleeing by sea.

Rajapaksa's youngest brother Basil, who resigned in April as finance minister, missed his own Emirates flight to Dubai early Tuesday after a tense standoff of his own with airport staff.

Basil - who holds US citizenship in addition to Sri Lankan nationality - tried to use a paid concierge service for business travellers, but the airport and immigration staff said they had withdrawn from the fast track service.

"There were some other passengers who protested against Basil boarding their flight," an airport official told AFP. "It was a tense situation, so he hurriedly left the airport."

Unity government

Basil had to obtain a new US passport after leaving his behind at the presidential palace when the Rajapaksas beat a hasty retreat to avoid mobs on Saturday, a diplomatic source said.

Official sources said a suitcase full of documents had also been left behind at the stately mansion along with 17.85 million rupees (about $50,000) in cash, now in the custody of a Colombo court.

There was no official word from the president's office about his whereabouts, but he remained commander-in-chief of the armed forces with military resources at his disposal.

Rajapaksa is accused of mismanaging the economy to a point where the country has run out of foreign exchange to finance even the most essential imports, leading to severe hardships for the 22 million population.

If he steps down as promised, Prime Minister Ranil Wickremesinghe will automatically become acting president until parliament elects an MP to serve out the presidential term, which ends in November 2024.

But Wickremesinghe has himself announced his willingness to step down if consensus is reached on forming a unity government.

The succession process could take between three days - the minimum time taken to convene parliament - and a maximum of 30 days allowed under the statute. If Rajapaksa does step down on Wednesday, the vote would take place on July 20, the parliamentary speaker has said.

The leader of the main opposition party Samagi Jana Balawegaya party, Sajith Premadasa, who lost the 2019 presidential election to Rajapaksa, has said he will stand for the position.

Premadasa is the son of former president Ranasinghe Premadasa, who was assassinated in a Tamil rebel suicide bombing in May 1993.

Sri Lanka defaulted on its $51-billion foreign debt in April and is in talks with the IMF for a possible bailout.

The island has nearly exhausted its already scarce supplies of petrol. The government has ordered the closure of non-essential offices and schools to reduce commuting and save fuel.