Thousands of protesters calling for the release of Pakistan's jailed ex-prime minister Imran Khan defied roadblocks and tear gas to march to the gates of the nation's capital on Tuesday.

The government said one police officer was killed and nine were critically wounded in two days of clashes with demonstrators as they closed in on Islamabad.

Khan was barred from standing in February elections that were marred by allegations of rigging.

He was sidelined by dozens of legal cases which he claimed were fabricated to prevent his comeback.

His Pakistan Tehreek-e-Insaf (PTI) party has defied a government crackdown with regular demonstrations, aiming to seize public spaces in Islamabad and other large cities.

The capital has been locked down since late Saturday, with mobile internet sporadically cut and more than 20,000 police flooding the streets, many armed with riot shields and batons.

Last week, the Islamabad city administration announced a two-month ban on public gatherings.

But PTI convoys travelled from their power base in northwestern Khyber Pakhtunkhwa province and the most populous eastern province of Punjab, hauling aside roadblocks of stacked shipping containers.

In the early hours of Tuesday, protesters were clashing with police who were firing tear gas and rubber bullets at a motorway entry to Islamabad.

PTI's chief demand is the release of Khan, the 72-year-old charismatic former cricket star who served as premier from 2018 to 2022.

He has been jailed since August 2023, facing legal accusations ranging from illegal marriage to graft and inciting riots. (AFP)