Sri Lankan president Gotabaya Rajapaksa has submitted a letter of resignation, two government sources said on Friday, after he fled to Singapore following mass protests over his country's economic meltdown.

Rajapaksa emailed a letter of resignation to the speaker of the country's parliament late on Thursday, two sources said.

It was not immediately clear if the letter, sent shortly after Rajapaksa arrived in Singapore, would be accepted in email form, the sources added.

In commercial capital Colombo, troops patrolled the streets to enforce a curfew.

Rajapaksa, who fled to the Maldives on Wednesday to escape a popular uprising over his family's role in a crippling economic crisis, headed on to Singapore on a Saudi Arabian airline flight, according to a person familiar with the situation.

A passenger on the flight, who declined to be named, told Reuters that Rajapaksa was met by a group of security guards and was seen leaving the airport VIP area in a convoy of black vehicles.

Airline staff on the flight told Reuters the president, dressed in black, flew business class with his wife and two bodyguards, describing him as "quiet" and "friendly".

Singapore's foreign ministry said Rajapaksa had entered the country on a private visit and had not sought or been granted asylum.

His decision on Wednesday to make his ally Prime Minister Ranil Wickremesinghe the acting president triggered more protests, with demonstrators storming parliament and the premier's office demanding that he quit too.

"We want Ranil to go home," Malik Perera, a 29-year-old rickshaw driver who took part in the parliament protests, said on Thursday. "They have sold the country. We want a good person to take over. Until then, we won't stop."

Protests against the economic crisis had simmered for months and came to a head last weekend when hundreds of thousands of people took over government buildings in Colombo, blaming the powerful Rajapaksa family and allies for runaway inflation, shortages of basic goods, and corruption.

Rajapaksa, his wife and two bodyguards fled the country on an air force plane early on Wednesday and headed to the Maldives.

Inside the president's residence early on Thursday, ordinary Sri Lankans wandered the halls, taking in the building's extensive art collection, luxury cars and swimming pool.

"The fight is not over," said Terance Rodrigo, a 26-year-old student who said he had been inside the compound since it was taken over by protesters on Saturday along with the prime minister's official residence.

"We have to make society better than this. The government is not solving people's problems."

The usual protest sites, however, were calm, and organisers handed back the president and prime minister's residences to the government on Thursday evening.

"With the president out of the country..., holding the captured places holds no symbolic value anymore," Chameera Dedduwage, one of the organisers, told Reuters.

But another organiser, Kalum Amaratunga, said a crackdown could be imminent after Wickremesinghe branded some protesters "fascists" in an address the previous evening.

The government imposed a curfew in Colombo from noon (2.30PM Malaysia time) on Thursday to early morning on Friday in a bid to prevent further unrest. Local media showed armoured vehicles with soldiers atop patrolling the city's streets.

The military said troops were empowered to use force to protect people and public property.

One dead, 84 hurt in clashes

Police said one person was killed and 84 injured in clashes between riot police and protesters on Wednesday near the parliament and prime minister's office, as people demanded the ouster of both Rajapaksa and Wickremesinghe.

The army said two soldiers were seriously injured when they were attacked by protesters near parliament on Wednesday evening and that their weapons and magazines were snatched.

Police said the man who died was a 26-year-old protester who succumbed after he was injured near the premier's office.

Former prime minister Mahinda Rajapaksa and former finance minister Basil Rajapaksa, both brothers of the president, informed the Supreme Court through their lawyer that they would remain in the country until at least Friday.

They were responding to a petition filed by anti-corruption body Transparency International seeking action "against persons responsible for the current economic crisis".

Immigration officials had stopped Basil Rajapaksa from flying out of the country on Tuesday.

Parliament is expected to name a new full-time president on July 20, and a top ruling party source told Reuters that Wickremesinghe was the party's first choice, although no decision had been taken. The opposition's choice is their main leader, Sajith Premadasa, the son of a former president.