They held separate marches in Colombo, shouting slogans which accused authorities of corruption and mismanagement, and later came together for joint rally.
Tissa Attanayake, an opposition lawmaker, said law and order has broken down in the country and the government has politicised judicial institutions.
The demonstrators also protested attacks on journalists who have been critical of the government, holding banners that read "Stop Suppressing Media."
Government spokesman Keheliya Rambukwella rejected the accusations, saying, "This is a democratic country and everyone has the right to demonstrate and protest."
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President Mahinda Rajapaksa has used his party's overwhelming majority in Parliament - gained through the popularity of winning a civil war against ethnic Tamil rebels to expand his power.
He abolished a two-term presidential limit, scrapped independent commissions and took over their power to appoint top judges and police.
His party also voted last year to oust the country's first female chief justice, and Rajapaksa appointed his own aide to that position.
Sunil Jayasekara, convener of the Free Media Movement, said his group joined the protest to highlight high-profile attacks on media institutions and journalists that have occurred in January in recent years.
Jayasekara said his group is not satisfied with the investigations of those attacks, in which no suspects have been arrested.
More than 80 journalists have fled Sri Lanka since 2005.
The government has been accused of failing to properly investigate a series of attacks on journalists who were viewed as critical of the administration.
The human rights group Amnesty International says at least 15 journalists have been killed in Sri Lanka since 2006.