LAHORE, Pakistan (AFP) - Thousands of protesters on Thursday began a march from the Pakistani city of Lahore to the capital in a bid to unseat the government, which they claim was elected by fraud.

Supporters of cricketer-turned-politician Imran Khan and populist preacher Tahir-ul-Qadri massed separately in Lahore before announcing the start of their 300km journey to rally in Islamabad.

Both Khan and Qadri say the May 2013 general election was rigged and want Prime Minister Nawaz Sharif to resign and hold new polls. Sharif won by a landslide.

The two groups, travelling in motorised convoy to Islamabad on the anniversary of Pakistan's independence from Britain, made slow progress in setting off and by late evening had not even left Lahore.

The marches are now expected to reach the heavily-guarded capital some time on Friday.

The authorities had insisted Qadri's Pakistan Awami Tehreek (PAT) march would not be allowed, but relented in the afternoon.

Khan, whose Pakistan Tehreek-e-Insaaf (PTI) political party came third in the polls, was earlier given the go-ahead for his "Azadi" (freedom) march.

Government officials have accused the march organisers of trying to derail democracy and Sharif said the marches were a distraction from more pressing issues.

Pakistan is currently waging a military offensive against Taleban hideouts in its northwest, while also trying to boost a sagging economy and solve a chronic power supply crisis.

"This is the real Azadi march, taking place here, both civil and military leadership is together here and celebrating Pakistan's independence day, what can be a bigger march than this?" said Sharif.

The Prime Minister spoke at a ceremony to reopen a house once owned by Pakistan's founding father Muhammad Ali Jinnah, alongside army chief General Raheel Sharif, in an apparent show of unity.

Security in Islamabad has been ramped up in recent days, with more than 20,000 police and security forces on the streets and almost all roads into the city blocked by the authorities with barbed wire and shipping containers.

After days of speculation that the authorities would seek to stop marchers entering the city, Interior Minister Chaudhry Nisar Ali Khan told reporters that both PTI and PAT would be allowed to hold rallies.

City officials were in talks with the two groups to work out details, he said.

Announcing the start of the march in Lahore, Qadri pledged a peaceful march and rattled off a list of promises including discounted water and gas, and justice and healthcare for every citizen.

PAT activists clashed with police last week when they tried to remove blockades around the cleric's residence, leaving at least two people dead, and there were fears of more violence on Thursday.

An AFP journalist saw PAT supporters equipping themselves with clubs, masks and helmets in preparation for the march.

There was a festive mood in Lahore's Zaman Park as thousands of PTI supporters gathered for the march waving green and red party flags, dancing and singing patriotic songs.

Khan urged his followers on, galvanising them into action.

"If you succeed, then there will be justice in Pakistan and people all around the world will respect the green passport," Khan told the cheering crowd.

Several thousand PTI supporters travelled to Islamabad from northwestern Khyber Pakhtunkhwa province, where the party holds power.

Khan, a cricket hero who led Pakistan to World Cup glory in 1992, has persistently cried foul over last year's election and tried numerous avenues to have the results of a number of seats thrown out.

But international observers who monitored the polls said they were free and credible and critics say PTI should not have accepted their seats in parliament if they did not believe the vote was fair.

Some have accused PTI and PAT of being aided by the powerful military establishment to undermine the government and shore up its own power.

On Monday, Qadri told AFP he wanted an "interim national government" consisting of technocrats and experts.

In a country which has seen three coups in its 67-year history, the threat of army intervention always hovers in the background at times of unrest.

Political analyst and author Imtiaz Gul said there was a "real danger of bloodshed and violence", but added a coup was unlikely, though the coming days could leave the civilian government weakened.

There have been rumours that elements within the military are unhappy with the way the Sharif government has pursued criminal charges, particularly a treason case, against former army chief Pervez Musharraf.