Thousands of people gathered in the Malawian capital Lilongwe on Thursday to demonstrate against the re-election of President Peter Mutharika, which protestors say was due to fraud.
About 3,500 marchers headed towards parliament where a two-day vigil is planned against the result of the May 21 presidential vote.
Many shops were closed and police were heavily deployed, AFP journalists saw.
In Blantyre, the financial capital, protesters said they had been attacked and forced to disperse by people wearing T-shirts showing support for the ruling Progressive Democratic Party (DPP).
Gift Trapence, of the Human Rights Defenders Coalition which organised the protests, accused DPP "thugs" of inciting violence.
"We are undeterred because we know that this is an attempt to distract us," Trapence said. "We have since taken up this issue with the police."
The planned two-day protest follows a string of demonstrations which have often been broken up by police firing tear gas.
The election was marred by allegations of fraud, including that many results sheets were altered using typewriter correction fluid.
The two main opposition parties have challenged the vote outcome in the court.
President Mutharika, leader of the DPP, held onto power by narrowly defeating Lazarus Chakwera of the MCP in the presidential vote.