Thousands of Haitian protesters were demanding the resignation of President Michel Martelly when riot police fired tear gas to disperse them.
Haiti has recently seen a series of protest marches amid growing public anger over the cost of living and claims of high levels of corruption.
Meanwhile, Mr Martelly appealed for unity in the impoverished nation.
He took office two years ago promising a fresh start for Haiti, devastated by an earthquake in 2010.
But Haiti – which shares the Caribbean island of Hispaniola with the Dominican Republic – remains one of the poorest countries in the western hemisphere.
On Monday, the protesters set up burning barricades of discarded tyres as they marched in the capital, demanding that Mr Martelly stand down immediately.
“We are moving forward to removing him from power and won’t stop until he leaves,” demonstrator Jean Daniel was quoted as saying by the Associated Press.
A number of demonstrators hurled stones at the police, who responded with tear gas.
There were also reports that gunshots were fired in Port-au-Prince, but it was not immediately known whether there were any injuries.
A rival pro-Martelly rally was also held in the capital, and two sides later confronted each other by throwing rocks.
In the northern city of Cap-Haitien, a smaller opposition demonstration was organised.
Monday’s protests were among the biggest since Mr Martelly took office two years ago.
Speaking in Cap-Haitien during a ceremony to mark Haiti’s final battle before it secured independence from France in 1804, the president made an impassionate plea for political unity.
“If we didn’t put our heads together, we wouldn’t have had the Battle of Vertieres,” he said.
“If we didn’t have our heads together, we wouldn’t have a Haitian state.”
The former pop star vowed after his election victory to bridge the gap between Haiti’s tragic past with the aspirations of a new generation.
But the opposition accuses him of wasting public money in luxury vehicles and international trips.
It also says the government has failed to hold legislative and local elections.
Mr Martelly denies the allegations, saying his government is addressing economic and political problems.
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