The United National Party (UNP) said repeated violence against the opposition, arson attacks on anti-government news organisations and arrests of student leaders could lead to protests similar to those sweeping the Arab world.
"The rulers cannot suppress the people forever," UNP deputy leader Karu Jayasuriya said in a statement. "We urge the government to look at what is happening in the Arab world and learn a lesson.
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Violence erupted in the capital on Friday night when ruling party activists attacked a UNP-led march demanding the release of their presidential candidate and former army chief Sarath Fonseka, who is serving a 30-month prison term.
At least four opposition MPs were among a dozen people wounded in the Friday night attack, while cars belonging to opposition lawmakers were smashed by ruling party activists.
"The two groups fought as the protest march went past a religious ceremony organised by some government lawmakers," police deputy inspector general Anura Senanayake said