Thousands gather for Pakistan marches

Thousands of anti-government protesters have begun a march on the Pakistani capital Islamabad amid fears that a prolonged confrontation might lead to a military coup a year after the country's first democratic transfer of power, officials say.

Former cricketing star Imran Khan was leading the protest to topple the government of Prime Minister Nawaz Sharif for failing to probe alleged fraud in last year's elections.

The marchers would cover more than 350 kilometres to reach Islamabad from the eastern city of Lahore, the political base of both Sharif and Khan.

The government had decided to allow Khan's march after assurances that his supporters would remain peaceful but it blocked another protest planned by a Pakistani-Canadian cleric Tahirul Qadri, fearing clashes.

The protests are seen a major test for a country where military coups are common.

Khan wants Sharif to step down, citing irregularities in the May 2013 election.

Qadri is demanding changes to the election system, which he says is undemocratic.

In a televised address to the country on Tuesday, Sharif offered to investigate allegations of election fraud. Khan rejected his overture.

The government has tightened security and blocked roads in Islamabad to prevent protesters from entering the city, raising fears of violence.

More than a dozen supporters of Qadri and at least three policemen were killed in protests ahead of the planned marches.

Political commentators said the army's reaction to the protests will decide Sharif's fate.

The army has ruled Pakistan for nearly half of its history since independence from Britain in 1947.