Pakistan police said on Wednesday they had arrested nearly 1,000 protesters who marched on the capital Islamabad demanding the release of jailed ex-prime minister Imran Khan, after crowds were evicted from the city centre in a sweeping security crackdown.

Khan has been jailed since August 2023, sidelined by dozens of legal cases he claims were concocted to prevent his comeback in elections this year marred by rigging allegations.

Since the February vote, his Pakistan Tehreek-e-Insaf (PTI) party has defied a government crackdown with regular rallies, but Tuesday's gathering was by far the largest to grip the capital since the poll.

More than 10,000 protesters surged into the city, defying a lockdown and a ban on public gatherings to skirmish with 20,000 security forces enlisted to turn them back.

Islamabad Police Inspector General Ali Nasir Rizvi said 954 protesters had been arrested between Sunday and Tuesday, when crowds came within 1.6 kilometres of a public square they aimed to occupy.

The government said at least one police officer and four state paramilitary personnel had been killed, before the main thoroughfare was cleared by forces armed with tear gas and batons early on Wednesday.

Interior Minister Mohsin Naqvi said in a statement that security forces had "bravely repulsed the protesters" as PTI told activists on social media the rally was cancelled "for the time being."

Khan made a statement from his cell outside Islamabad calling supporters to the capital on Tuesday.

But the crowds were led by his key lieutenant Ali Amin Gandapur and his wife Bushra Bibi, who was also jailed earlier this year but released last month.

"The movement is continuing and it will be ended only by Imran Khan," said Gandapur after retreating from Islamabad to northwestern Khyber Pakhtunkhwa province where he serves as chief minister.

Prime Minister Shehbaz Sharif called the protests "extremism." Since Sunday, his ministers have held regular press conferences in central Islamabad vowing no mercy for the oncoming marchers. (AFP)