Malaysian riot police have fired tear gas and used water cannon on a crowd of protesters who had converged on the centre of the capital, Kuala Lumpur.
Some 25,000 protesters marched on a central square in the city calling for changes to the electoral system.
They believe the current arrangements will benefit Prime Minister Najib Razak's long-ruling coalition in forthcoming elections.
The rally is one of the largest in the country in the past decade.
Police had sealed off the city's historic Merdeka (Independence) Square with barbed wire and barricades and began firing tear gas when these were breached by the demonstrators.
The demonstration was organised by an opposition-backed reform group, Bersih.
Malaysia's parliament approved a series of electoral reform measures this month, but activists say these do not address the main issue of electoral fraud which they maintain has kept the ruling Barisan Nasional coalition in power since independence.