The rally near Parliament comes amid a constitutional crisis sparked by President Maithripala Sirisena's move to oust Prime Minister Ranil Wickremesinghe, replace him with ex-leader Mahinda Rajapaksa, and suspend Parliament.
Supporters at the rally chanted "Whose power is this? Mahinda's power!"
Wickremesinghe has refused to vacate his official residence claiming he is the lawful prime minister. Thousands of his supporters have been keeping vigil.
Critics say the suspension of Parliament was meant to give Rajapaksa time to gather enough support to survive a no-confidence vote when lawmakers reconvene Nov. 14.
Wickremesinghe said in an interview with The Associated Press last week that there is credible evidence that Rajapaksa's party is attempting to buy support in Parliament. Palitha Range Bandara, a United National Party lawmaker, has said that he was offered millions of dollars and a minister portfolio if he crossed over.
Lawmakers from Rajapaksa's party have denied the allegations.
Seven members from Wickremesinghe's United National Front have defected to Rajapaksa's government.
On Saturday, the Tamil National Alliance -an ethnic minority Tamil party- said that it will support a no-confidence motion to be brought against Rajapaksa, after one lawmaker from their party joined Rajapaksa's government.
The Tamil party's 15 votes could give Wickremesinghe's camp a decisive edge over Rajapaksa.
After sacking Wickremesinghe, Sirisena announced that he made the replacement in part because Wickremesinghe and a Cabinet colleague were behind an alleged assassination plot against him.
Details of the alleged plot have not been disclosed and Wickremesinghe has repeatedly denied the accusation.
Tensions had been building between Sirisena and Wickremesinghe for some time, as the president did not approve of economic reforms introduced by the prime minister.
Sirisena was also critical of investigations into military personnel accused of human rights violations during Sri Lanka's long civil war, which ended in 2009.
Rajapaksa is credited as a hero by Sri Lanka's ethnic Sinhalese majority for ending the conflict.