COLOMBO, July 14 (Reuters) - Sri Lankan president Gotabaya Rajapaksa submitted a letter of resignation on Thursday, a spokesperson for the parliament speaker said, hours after fleeing to Singapore following mass protests over an economic meltdown.

The announcement triggered jubilation in the commercial capital Colombo where protesters massed outside the presidential secretariat, defying a city-wide curfew.

Crowds set off firecrackers, shouted slogans and danced ecstatically at the Gota Go Gama protest site, named mockingly after Rajapaksa's first name.

"The whole country will celebrate today," Damitha Abeyrathne, an activist, said. "It's a big victory."

"We never thought we would get this country free from them," she added, referring to the Rajapaksa family who dominated the South Asian country's politics for two decades.

Rajapaksa submitted his resignation by email late on Thursday and it would become official on Friday, once the document had been legally verified, the speaker's spokesperson said.

Rajapaksa fled to the Maldives on Wednesday then headed on to Singapore on Thursday on a Saudi Arabian airline flight, according to a person familiar with the situation.

The Maldives government confirmed late on Thursday that the country had granted diplomatic clearance for a Sri Lanka Air Force aircraft that carried President Gotabaya Rajapaksa and his spouse, on a transit visit.

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.

Rajapaksa's 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 earlier on Thursday. "They have sold the country, we want a good person to take over, until then we won't stop."