Pakistan's ousted prime minister Imran Khan on Wednesday led a convoy of thousands of supporters towards the capital Islamabad in a show of force the new government has attempted to shut down, with clashes breaking out between police and protesters.

Since being removed from power through a no-confidence vote last month, the cricket star turned politician has heaped pressure on the country's fragile new coalition rulers by staging mass rallies.

In a centrepiece showdown with his rivals, Khan had called for supporters to march towards the capital and stage a sit-in until fresh elections are called.

"No obstacle can stop us, we will cross all the barriers and will reach... Islamabad," Khan declared from atop a truck.

He made a dramatic arrival in a helicopter, touching down on a motorway clogged with hundreds of cars and walking supporters outside the city of Mardan, 100 kilometres (62 miles) northwest of Islamabad.

As the rotors wound down, the 69-year-old leant out of the door to wave at the crowd, before disappearing into a chaotic throng waving the red-and-green flags of his Pakistan Tehreek-e-Insaf (PTI) party.

All roads surrounding Islamabad were blocked, with a heavy security presence in place, while key entry and exit points outside nearby major cities including Lahore, Multan and Peshawar were also shut.

Police deployed tear gas against hundreds of Khan backers attempting to remove roadblocks in Lahore and Faizabad, where PTI supporters also seized an excavator to clear the route of containers.

Yasmin Rashid, a prominent PTI leader, told local media police smashed the windows of her car as she tried to drive from Lahore to Islamabad.

"We will reach Islamabad at all costs. We will deal with any obstructions... and follow the orders of our leader," shopkeeper and protester Irfan Ahmad, 34, told AFP in Peshawar.

MAJOR DISRUPTION

The coalition government headed by Prime Minister Shehbaz Sharif has pledged to stop Khan's supporters from pouring into the city, calling the rally an attempt to "divide the nation and promote chaos".

"Nobody should be allowed to besiege the capital and dictate his terms," interior minister Rana Sanaullah said Tuesday.

The Supreme Court on Wednesday held an emergency hearing over whether the government had legal grounds to block the rally.

Schools in the capital and neighbouring Rawalpindi were closed and all exams cancelled, while a state of emergency was declared at all hospitals, with staff put on alert.

"We have seen the capital blocked before but this is something unprecedented," Islamabad private school worker Allah Ditta, 52, told AFP.

Salon worker Sawera Masih complained that the wide-scale disruption was falling hardest on daily wage workers like herself.

"Whoever is in power doesn't make a difference to us, but not earning even for a single day affects me and my family," the 23-year-old said.

POLICE RAIDS

On Tuesday, PTI accused police of arresting and detaining hundreds of its supporters in overnight raids.

Police sources in Lahore who asked not to be named told AFP more than 200 supporters were detained on public order offences.

The government and police said protesters had been planning to join the march with weapons.

One police officer was shot dead during the raids, Punjab Chief Minister Hamza Shahbaz Sharif said.

Khan, who claims he was removed by a US-led foreign conspiracy, was voted in by an electorate weary of the dynastic politics of the country's two major parties.

The popular former sports star -- who enjoyed the backing of the country's powerful military -- had promised to sweep away decades of entrenched corruption and cronyism but is believed to have fallen out with Pakistan's generals.

He was brought down in part by his failure to rectify the country's dire economic situation, including its crippling debt, shrinking foreign currency reserves and soaring inflation.