Protesters pelted police with stones, beat up pedestrians and television cameramen, and set government vehicles on fire. Several policemen and demonstrators were injured in the street battles.
Sunday's protest came two days after the Maoists ended a prolonged nationwide strike, bowing to pressure from angry citizens, Western diplomats and business groups.
The Maoists traditionally back strike calls with the threat of violence against those who defy them.
The Maoists won the country's last elections and briefly led a coalition government. A dispute split their coalition, allowing the current government to come to power.
The Maoists now want the power back, but the government has refused to step aside.