Several thousands of activists belonging to the main opposition UNP, members of its allies, trade union and civil society groups staged a rally at Hyde Park to highlight government misdeeds.
The former army chief Sarath Fonseka, who heads his own party, held a separate march.
Leading Monk Venerable Maduluwawe Sobhitha said his move to join the opposition movement was meant to change the present system of governance. "We want an end to the executive presidency and not a mere change of government," he said.
The protests took place as the government has opted to hold two provincial council elections in March as means of testing its popularity.
More From This Section
The opposition charged that the government was attempting to make political capital out of an anticipated adverse Sri Lanka resolution at the UN Human Rights Council.
The government in turn accused the opposition parties of aiding and abetting the devious designs of the West.
President Mahinda Rajapaksa, who gained popularity after defeating ethnic Tamil Tiger rebels, had expanded his power by abolishing a two-term presidential limit, scrapping independent commissions and taking over their power to appoint top judges and police.