Opposition parties rallied to protest what they said were irregularities in last week's presidential election that gave incumbent Mahinda Rajapaksa a second term.
Losing candidate and former army commander General Sarath Fonseka vowed to campaign against the irregularities which he said were committed throughout the elections until the results were announced.
"I believe that I am still the people's president, though I was not declared the winner," Fonseka told the rally.
Commissioner of Elections Dayananda Dissanayaka said Wednesday that the results were not manipulated. Rajapaksa received 58 percent of the vote against 40 percent for Fonseka.
The main opposition parties said they would remain together to try to defeat Rajapaksa's ruling United People's Freedom Alliance in the parliamentary elections due to be held before April.
They also said they would file a petition for a review of alleged irregularities including misuse of state media, use of public officials and vehicles for the campaign and reports that some opposition poll watchers were threatened and chased away from ballot-counting centres.
The election commissioner conceded that there were irregularities during the campaign, but did not name any responsible parties.
Independent monitors made a series of allegations about the use of state-run television and radio stations for Rajapaksa's campaign, as well as state property such as vehicles and buildings.
It was the first election since the May 2009 defeat of the Liberation Tigers of Tamil Eelam.