Sri Lanka's president was considering using a navy patrol craft to flee the island Tuesday following a humiliating standoff with airport immigration, official sources said.

Gotabaya Rajapaksa has promised to resign on Wednesday and clear the way for a "peaceful transition of power" following widespread protests against him over the country's worst economic crisis.

The 73-year-old leader fled his official residence in Colombo just before tens of thousands of protesters overran it on Saturday. He then wanted to travel to Dubai, officials said.

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

But immigration officers refused to go to the VIP suite to stamp his passport, while he insisted he would not go through the public facilities, fearing reprisals from other airport users.

The president and his wife spent the night at a military base next to the main Bandaranaike International airport after missing four flights that could have taken them to the United Arab Emirates.

Rajapaksa's youngest brother Basil, who resigned in April as finance minister, missed his own Emirates flight to Dubai early Tuesday after a similar standoff 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 airport and immigration staff said they were withdrawing from the fast track service with immediate effect.

"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."

Hasty retreat

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 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.

A top defence source said the president's closest military aides were discussing the possibility of taking him and his entourage overseas aboard a naval patrol craft.

A navy boat was used on Saturday to take Rajapaksa and his aides to the northeastern port city of Trincomalee, from where he was helicoptered back to the international airport on Monday.

"The best option now is to take the sea exit," the defence official said. "He could go to the Maldives or India and get a flight to Dubai."

Another alternative, he added, would be to charter a plane to fly him from the country's second international airport at Mattala, opened in 2013 and  named after the president's elder brother Mahinda.

It is widely considered a white elephant, with no scheduled international flights and described as probably the world's least-used international airport.

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.

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.