Sri Lanka’s acting president has imposed a national curfew and told the military to “do whatever is necessary to restore order” as the island appeared on the brink of anarchy.

Protesters who forced President Gotabaya Rajapaksa to flee to the Maldives shifted their attention to his acting replacement Prime Minister Ranil Wickremesinghe.

The decision to leave Mr Wickremesinghe in charge triggered more demonstrations, with protesters storming the PM’s office and demanding that he go too and attempting to break into the homes of army, navy and airforce commanders.

Mr Wickremesinghe declared a state of emergency and imposed a curfew ordering that no person be on a public road, railway, public park, public recreation ground or other public ground or the seashore until Thursday morning.

Authorities declined to reveal the whereabouts of Mr Wickremesinghe.

Demonstrators surround Prime Minister Ranil Wickremasinghe’s desk. Photo: Getty

In a televised address, Mr Wickremesinghe called on protesters who have flooded the capital Colombo all week to leave his office and other state buildings and co-operate with authorities.

“We can’t tear up our constitution. We can’t allow fascists to take over. We must end this fascist threat to democracy,” he said.

Parliament is expected to name a new full-time president next week, and a top ruling party source told Reuters Mr Wickremesinghe was the party’s first choice, although no decision had been taken.

But an attempt by Mr Wickremesinghe to cling on would infuriate the protesters who say he is a close ally of the President Gotabaya Rajapaksa’s family which has dominated the country since 2005.

President Rajapaksa was driven out this week after an economic collapse unleashed an uprising on the streets and protesters stormed the presidential palace.

After escaping to the Maldives he’s expected to head to Singapore where media report he was likely to receive asylum.

Protesters storm the compound of prime minister’s office. Photo: Getty

The appointment of Mr Wickremesinghe has not quelled the anger of thousands of protesters who have continued to target key public buildings.

Police stationed outside his office fired several rounds of tear gas but the protesters were not deterred and surged into the compound.

“It feels pretty marvellous, people were trying to take this place for about three hours,” said college student Sanchuka Kavinda, 25, standing next to a mangled, open gate of the prime minister’s office.

“No matter what, everyone in this crowd will be here until Ranil also steps down.”

In a statement, Mr Wickremesinghe said the protesters had no reason to storm his office.

“They want to stop the parliamentary process. But we must respect the constitution,” he said.

Police use a large amount of tear gas and water cannon to disperse the crowd. Photo: Getty

On the lower floor of the whitewashed colonial-era building, dozens of protesters sang Sinhala pop songs.

A large group of security personnel armed with assault rifles sat in a room.

Protest organisers and security personnel manned a central wooden staircase at the heart of the building, guiding sightseers to and from the upper floor where the prime minister’s room is located.

At an adjoining room on the top floor, the plush furniture had been hastily pushed to the corners and a line of armed security personnel ushered visitors through.

Despite his flight, Mr Rajapaksa’s own resignation was not yet confirmed by late Wednesday.

Earlier, the parliament Speaker, Mahinda Yapa Abeywardena, said Mr Rajapaksa had phoned him and told him his resignation letter would arrive later on Wednesday.

An aide to Mr Abeywardena had no update on the letter late in the day.

Protests against the economic crisis have simmered for months and came to a head last weekend when hundreds of thousands of people took over government buildings in Colombo, blaming the Rajapaksas and their allies for runaway inflation, shortages and corruption.

Government sources and aides said the president’s brothers, former president and prime minister Mahinda Rajapaksa and former finance minister Basil Rajapaksa, were still in Sri Lanka.

On Tuesday, Sri Lankan immigration officials prevented Basil Rajapaksa, who quit in April as finance minister, from flying out of the country.

Mr Wickremesinghe, whose private residence in Colombo was set ablaze on Saturday, had offered to resign as prime minister but did not repeat that offer after he became acting president on Wednesday.

If he does go, the Speaker would be acting president until a new president is elected on July 20 as scheduled.

-with AAP