Cricket legend Imran Khan is expected to unveil his manifesto to become Pakistan's next prime minister at an election rally on Saturday, where turnout will be a key test for his chances of success.

Thousands of people poured towards Pakistan's independence monument in Lahore, the country's second largest city and Khan's hometown, waving the red and green flags of his Pakistan Tehreek-e-Insaaf (PTI) or Movement for Justice party.

Young men and women, his core support base, dominated the crowd, wearing "Imran Khan" and "Revolution" T-shirts, and caps in the party colour.

They carried pictures of Khan, who became an icon for captaining Pakistan's cricket team to its only World Cup win in 1992 and setting up a cancer hospital that provides world-class care, free of charge to the poor.

Khan chose the venue, where Indian Muslims made their first official demand for a separate homeland, and the timing -- the 73rd anniversary of the demand -- to kickstart his campaign just days after elections were called for May 11.

"We want a change," said Abdul Rehman, who travelled from the northwestern city of Peshawar which has been on the frontline of Taliban attacks and where Khan also attracts some support because of his ethnic Pashtun background.

"We are fed up with the old faces. They are the symbol of the status quo. We want to bring new faces and Imran Khan should come to power now," said another young man from Gujrat in Punjab province.

The young and the urban middle-class have been particularly drawn to Khan's determination to stamp out corruption, which is endemic in public life, tackle poverty and unemployment, and end the country's appalling power crisis.

They see PTI, founded in 1996, as a fresh alternative to the outgoing Pakistan People's Party and opposition Pakistan Muslim League-N of former prime minister Nawaz Sharif which have dominated civilian governments for decades.

Khan is a new breed of politician. While expensively educated and from a comfortable family, his achievements chalk him out as self-made compared to the feudal and industrial magnates of Pakistan's corrupt elite.

The party has become the first in Pakistan to hold internal elections, staking a claim to being the only truly democratic party in the country.

But critics say Khan is short on detail and warn that his chances of winning his first ever election campaign are practically nil.

In the last year, Khan's stock has dipped markedly in opinion polls and analysts say at best he can become a post-election kingmaker in any future coalition.

Liberals are also terrified by his reluctance to criticise the Taliban and his rhetoric against US drone strikes targeting Al-Qaeda, warning that he represents a dangerous flirtation with extremists.

Khan drew crowds of around 100,000 at an enormous rally in Lahore in October 2011, kickstarting what he has called a "tsunami", and will need to top that to prove that he is a contender in the race.

PTI is live-streaming the rally on the Internet, a medium that the party has tapped into heavily through social networking sites Facebook and Twitter.

Authorities have beefed up security for the rally and diverted traffic. They also shut mobile phone networks at the venue and immediate surrounding area as a precaution to guard against bombs, some of which are detonated by telephones.