Forcing their way into Islamabad after pushing the security forces on backfoot in violent clashes that resulted in at least half a dozen deaths earlier in the day, thousands of Pakistan Tehreek-e-Insaf (PTI) protestors have reached Islamabad's Zero Point 'D-Chowk' where they plan to hold an indefinite sit-in until party chief and former Pakistan Prime Minister Imran Khan is released from jail.

Pakistani protesters demanding the release of ex-prime minister Imran Khan on Tuesday killed four members of the nation's security forces, the government said, as the crowds defied police and closed in on the capital's centre.

Thousands of demonstrators supporting former Pakistani Prime Minister Imran Khan broke through barricades around Islamabad on Tuesday, killing four security service members and moving toward the center of the city.