Violent protests erupted in La Paz on Tuesday night over the disputed result of Sunday's presidential election.

Demonstrators clashed with police, who responded with tear gas and pepper spray.

Opposition supporters suspect foul play from the incumbent, Evo Morales, after the preliminary count was halted with over 80 per cent of the votes verified.

At that moment, it looked as if there would be a run-off, with Morales failing to secure an outright majority or hold a 10 per cent lead over his main rival Carlos Mesa.

However, the quick count was stopped for 24 hours, after which the Bolivian president had secured victory.

The opposition candidate quickly declared the results fraudulent in a speech to supporters.