Sri Lanka's President Gotabaya Rajapaksa sent his resignation letter by email on Thursday, a day after he fled to the Maldives with his wife and two bodyguards, escaping mass protests against him.
Mr Rajapaksa is now in Singapore, AP cited Maldives officials as saying.
Officials who spoke to Reuters on condition of anonymity said the Speaker of Parliament's office was expected to officially accept the resignation soon.
Mr Rajapaksa made his ally, Prime Minister Ranil Wickremesinghe — who had also said he would resign — the acting president, triggering further demonstrations as protesters stormed his office, demanding his removal.
At least 45 people were taken for medical treatment following a standoff between riot police and protesters on Wednesday night near Parliament, as Sri Lankans demanded the removal of both Mr Rajapaksa and Mr Wickremesinghe, hospital sources told Reuters.
Both men are facing a popular uprising against an economic crisis blamed on government mismanagement.
Local media said a protester, 26, who was taken to hospital after being tear-gassed, died of breathing difficulties.
"Some of the protesters who had gathered outside the Parliament building have now dispersed," police spokesman Nalin Thalduwa told Reuters. "The area is now calm."
An overnight curfew imposed by the acting president ended early on Thursday with no arrests, Mr Thalduwa said. 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.
Mr Rajapaksa left Colombo in a military plane early on Wednesday. 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 Rajapaksa from flying out of the country on Tuesday.
Sri Lanka's Parliament is expected to name a new full-time president next week, and a senior ruling party source said Mr Wickremesinghe was the party's first choice.
Protests against Sri Lanka's worst economic crisis since independence from Britain in 1948 have simmered for months.
It 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.