Opposition leaders submit no-trust motion in the National Assembly Islamabad on Tuesday. Twitter photo

The move comes after the opposition, led by the Pakistan Peoples Party (PPP), rallied thousands of supporters to demonstrate against Imran, raising the prospect of political turmoil in the nuclear-armed nation.

"Resign in 24 hours and face us in an election," PPP leader Bilawal Bhutto Zardari, the son of assassinated former prime minister Benazir Bhutto, told the rally just outside the capital Islamabad. "Or be prepared for a no-confidence move."

The opposition later submitted a formal request requiring Imran Khan to seek a parliamentary vote of confidence. The opposition needs a simple majority of 172 and says it requires just 11 more votes to force Imran out.

"We will have more than 172 votes," said Bilawal Bhutto's father, Asif Ali Zardari, a former president.

Opposition lawmakers and political analysts say Imran Khan, a former cricket star, has lost the backing of the powerful military whose support they say secured the path to power for his upstart political party four years ago.

Imran denies the military helped him into office. The military says it does not interfere in politics.

The speaker of the lower house must now convene parliament within two weeks, though it could take weeks before a vote takes place.

Imran vowed to fight any move to unseat him. "Whatever they do, I'm ready for that," he told a gathering in Islamabad.

Opposition leaders say Imran has lost public support as he struggles with high inflation, a rising current account deficit and depleting foreign reserves.

Imran has responded to economic problems with cuts in fuel and electricity prices, while rejecting calls to step down and warning the opposition of unspecified consequences if they force a vote of no-confidence.

Both the opposition and Imran Khan's party are riven by factions. Imran won a confidence vote last year by six votes. Pakistan's next general election is due by 2023.

Reuters