Supporters of Mr Khan have poured onto his march route enthusiastically. Photo: Reuters
Clashes have erupted in Pakistan after an attack on a convoy of protesters heading to the capital Islamabad, led by opposition politician Imran Khan.
A crowd pelted the convoy with stones in the city of Gujranwala. Officials from Khan's party said his vehicle was also shot at but he was not hurt.
This protest is being seen as the strongest challenge to Prime Minister Nawaz Sharif since he took office.
Anti-government cleric Tahirul Qadri is also leading a convoy to the capital.
In a post on its Facebook Page, Khan's PTI party alleged that a group of supporters of the governing PML-N party also fired on the huge bullet-proof container carrying Khan.
The reports of the shots are yet to be independently confirmed.
The government has denied that its party workers instigated any violence and denounced the marches as an attempt to derail democracy.
Nawaz Sharif took office last year in what was Pakistan's first peaceful democratic transfer of power from one civilian government to another.
ANGER OF PROTESTERS
Earlier, Khan addressed supporters in Gujranwala, where he paid tribute to the energy of the demonstrators.
"I am going to Islamabad and I will ensure that all of us are free," he was quoted as saying by Dawn newspaper.
The BBC's Shahzeb Jillani in Islamabad says that panic gripped the rally when Khan's convoy came under attack.
Scores were injured as the crowd ran to find cover and retaliate. Eyewitnesses said hundreds of policemen were deployed for security but failed to prevent the clash.
Imran Khan has said the incident will not deter his supporters from marching on to Islamabad to demand the government's resignation.
He told his followers not to fall for what he described as a trap to instigate violence.
Thousands of Qadri's supporters have also been galvanised to make the journey to Islamabad.
Both protest camps want the prime minister to resign. They are angry about the sinking economy, growing militancy, and failure to deliver core services such as a steady electricity supply.
Khan has also accused Sharif of failing to probe fraud in last year's polls.
But in a country with a history of military coups, there is a fear of violence and of a possible army reaction, correspondents say.