Pakistan's military gave residents of North Waziristan until the end of the day to leave the remote mountainous region ahead of a widely anticipated major ground offensive by the army against Islamist militants.

At least 430,000 people have fled the region into nearby areas of Pakistan as well as neighbouring Afghanistan, the biggest movement of refugees in Pakistan in years.

The military has sent fighter jets to flush out Taliban militants at the start of a comprehensive operation after a brazen attack this month on Karachi airport, Pakistan's largest.

But the ground offensive has yet to start.

"Today is the last day for the people to leave the tribal region," a military official told Reuters by telephone from the North Waziristan capital of Miranshah.

"The curfew will be then imposed and preparations made for the ground offensive."

Many of those who stayed behind - their number is unknown - said they could not afford to pay for cars to take them to safer places such as Bannu, a dusty town on the edge of the region, where most refugees have settled.

Other residents complained the government was not doing enough to help them. Many opted to stay with relatives rather than official camps.

Some of the region's most feared al Qaeda-linked militants are holed up high up in the North Waziristan mountains, using the area as a launching pad for attacks within Pakistan as well as against NATO forces in neighbouring Afghanistan.

Pakistan has for years been under US pressure to do more to eliminate these strongholds.

But the government of Prime Minister Nawaz Sharif has instead insisted on trying to engage militants in peace negotiations.

Mr Sharif's peace initiative collapsed after the attack on Karachi airport, a turning point that convinced the government to abandon peace talks and announce military action.

Refugees said settlements in North Waziristan had been reduced to ghost towns.

Meanwhile, a prominent cleric returned to Pakistan today to lead what he calls a peaceful revolution against Mr Sharif as his supporters fought pitched battles with police firing tear gas in the capital Islamabad.

Tahir ul-Qadri, a Pakistani preacher turned political activist who lives in Canada, is a divisive figure in Pakistan, where he made headlines last year by leading mass rallies against the previous government.

As a plane carrying the cleric approached Benazir Bhutto International Airport, violence broke out on the ground as police fired tear gas at 2,000 of his supporters.

The authorities, fearing an escalation of unrest, diverted the commercial flight to the eastern city of Lahore.

Mr Qadri's sudden ascent to prominence has prompted speculation that the army, which ruled Pakistan for decades, may be using him as a proxy in efforts to sideline the civilian government.

Even after the aircraft landed in Lahore, Mr Qadri and his supporters refused to leave the plane for hours, demanding it fly back to Islamabad or for the army to send a representative to protect him.

He eventually disembarked and was escorted to his residence in Lahore, his main base in the country.

Outside Lahore airport, about 1,000 supporters held a peaceful rally.

Mr Qadri's spokesman, Shahid Mursaleen, said he wanted to stage an Arab spring-style revolt and install a government that would enforce reform, tackle terrorism and improve accountability.