Thousands of supporters of Pakistan's imprisoned former Prime Minister Imran Khan have defied a lockdown and widespread arrests to head to the capital Islamabad to demand his release.

Hundreds of them reached the outskirts of the city on Monday, his party and officials said, amid reports of violence elsewhere.

Authorities said at least one policeman was killed and dozens of officers were injured in clashes just outside of Islamabad.

Police said tear gas had been used in an attempt to halt the marchers about 25 kilometers (15 miles) from the city. 

The protest march, which Khan has described as the "final call," is one of many his party had held to seek his release since he was jailed in August last year.

Islamabad on lockdown as Khan supporters rally

The rally comes after the authorities enforced a security lockdown over the last two days to block the protests.

Highways into the city have been barricaded, gatherings have been banned and mobile and internet access has been blocked "in areas with security concerns," the Pakistani Interior Ministry said.

They did not specify the areas or how long the measures would be in place.

Officials and witnesses said all public transport between cities and terminals had also been shut down in the eastern province of Punjab to keep away the protesters, led by members of Khan's Pakistan Tehreek-e-Insaf (PTI) party.

According to Punjab provincial government Information Minister Uzma Bukhari, about 80% of Khan's supporters had been arrested. Local police said it has arrested 4,000 Khan supporters, including five parliamentarians.

"We wouldn't let them storm the capital," he said.

PTI says Pakistani government using violence to suppress protesters 

Islamabad saw an extra layer of security being added ahead of a visit by Belarus leader Alexander Lukashenko, who arrived on Monday for a three-day visit. He was received at an airport near the capital by Prime Minister Shehbaz Sharif.

According to the PTI, the current government is using violent tactics to block the protesters, saying it had arrested hundreds of workers and leaders.

"They are even firing live bullets," one of Khan's aides, Shaukat Yousafzai, told Pakistani broadcaster, Geo News TV.

The party's most recent protest in Pakistan's capital, which took place in October, turned violent.

Khan was voted out of power by parliament in 2022 after falling out with Pakistan's powerful military. He faces charges ranging from corruption to instigation of violence, all of which he and his party deny.

kb,ftm/wd (Reuters, AP)