Islamabad - Political tension was high in Pakistan on Thursday as thousands protested an overnight court ruling that put an election ban on the country's top opposition leaders, leading to the imposition of federal rule in the most populous province of Punjab.

The Supreme Court on Wednesday barred former prime minister and the country's most popular leader, Nawaz Sharif, from running for parliament and nullified last year's election of his brother, Shahbaz, to the Punjab Assembly, removing him from the seat of chief minister.

The two brothers alleged that President Asif Ali Zardari was behind the court verdict, widening political rifts between the two traditional rivals, who allied briefly after February 2008 elections to form a coalition government and successfully oust the former military strongman Pervez Musharraf.

Following the court ruling, Zardari imposed federal rule and stopped the regional assembly from holding a session in Punjab, where Sharif's Pakistan Muslim League-Nawaz (PML-N) is in power, apparently in a bid to block PML-N from choosing a new chief minister.

Provincial legislators from Sharif's party gathered outside the Punjab Assembly in the eastern city of Lahore on Thursday, but a battery of police did not allow them to enter the building that was also locked to prevent holding a session.

"Locking the lawmakers out of the house is a shameful act," Punjab Assembly Speaker Rana Mohammad Iqbal told reporters, as scores of lawmaker's chanted slogans.

Iqbal later started a noisy "session" on the stairs leading to the building entrance, claiming that holding a meeting was their constitutional right that cannot be taken away through any law.

Provincial Minister Rana Sanaullah said riot police deployed outside the barricaded assembly building arrested at least a dozen demonstrators.

Anti-government rallies were also held in other cities of Punjab in which protestors lambasted Zardari and denounced the court decision. They burnt tyres and tore off banners carrying pictures of Zardari and other leaders of his Pakistan People's Party.

A minor clash between the charged political activists and the law enforcers was reported in the garrison town of Rawalpindi, which adjoins the capital city Islamabad.

The PML-N has given a call for countrywide protests against Sharif's disqualification because of his conviction on several charges, including plane hijacking, after the then army chief Musharraf toppled his government in a military coup in 1999.

Sharif was later pardoned but exiled for more than seven years, only to return few weeks before the February 2008 polls. His party emerged as the second largest and forged an alliance with Zardari's Pakistan People's Party to form a coalition government.

But he had to part ways with Zardari after he refused to reinstate the independent-minded chief justice Iftikhar Chaudhry, who was sacked by Musharraf in November 2007 in a bid to avoid disqualification for a second term in the president's office by the court.

The brewing political crisis has also set alarm bells ringing for Western governments as they want Pakistan to focus on the fight against al-Qaeda and the Taliban who have sanctuaries on vast swathes of ungoverned land in the north-west.

However, the US government has publicly said that the electoral ban on Sharif and his brother "is an issue for Pakistan to deal with." (dpa)