Sri Lanka's president submitted his resignation shortly after reaching Singapore on Thursday, the parliamentary speaker's office said, days after the head of state fled protests triggered by his country's worst-ever economic crisis.

Gotabaya Rajapaksa's emailed resignation would be examined before a formal announcement – expected on Friday – is made, the speaker's spokesman Indunil Yapa said.

Rajapaksa fled Sri Lanka on Wednesday, after protesters overran his palace on the weekend, heading first to the Maldives and then Singapore.

"The authenticity and the legality of the e-mail will have to be checked out" before being formally accepted, Yapa said.

Rajapaksa would be the first president to resign since Sri Lanka adopted a presidential system of government in 1978.

A small but jubilant crowd, some waving the national flag, danced and chanted in celebration outside the presidential secretariat as news of the resignation broke.

"This is a monumental victory," said protester Harinda Fonseka. "But it is only a first step."

Under Sri Lanka's constitution, Prime Minister Ranil Wickremesinghe – whose resignation is also being demanded by protesters – would automatically become acting president until parliament can appoint a successor.

Rajapaksa, his wife Ioma and their two bodyguards arrived in Singapore from Male on board a Saudia airline flight.

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

The former Maldivian president Mohamed Nasheed is believed to have played a behind-the-scenes role in getting him out of the country, and said Rajapaksa feared he would be killed if he remained.

"I believe the President would not have resigned if he were still in Sri Lanka, and fearful of losing his life," Nasheed tweeted. (AFP)