LAHORE (REUTERS) - Thousands of protesters began to march on the Pakistani capital from the eastern city of Lahore on Thursday, buoyed by a last-minute court order that a peaceful march could go ahead and a government promise to obey the ruling.

The festive air at the home of cricketer-turned-opposition politician Imran Khan was in stark contrast to the grim determination at the blockaded home of cleric Tahir ul-Qadri, another would-be protest leader whose march has been banned.

Khan and Qadri are not officially allied though both are calling for the ouster of a government they condemn as corrupt, which came to power after a sweeping general election victory for the party of Prime Minister Nawaz Sharif last year.

"We are the ones who pose the biggest challenge to the government, that is why they are opposing us so strongly," Qadri's spokesman, Shahid Mursaleen, said late on Wednesday. "The police are killing us and our people only have sticks to protect themselves."

The protests have raised tension in the nuclear-armed country of 180-million people and have revived concern about the central issue in Pakistani politics: competition for power between the military and civilian leaders.

Any threat to Pakistan's stability alarms its allies and neighbours, who fear rising religious intolerance and the Islamist militants who find refuge there.

Some officials had accused elements within the powerful military of orchestrating the protests to weaken the civilian government. The military has declined to comment but has previously said it does not meddle in politics.

Many analysts doubt whether the military wants to seize power, but there is a widespread perception it could use the opportunity to put the civilian government under its thumb.

Sharif is relying on the military for security in the face of the challenges, and, as a result, the government is likely to be less determined to pursue polices the military objects to, such as the prosecution on treason charges of former military leader Pervez Musharraf, analysts say.

By early on Thursday, the government appeared to have developed a strategy to blunt the challenge to its power with the contrasting approaches to the two marches neatly splitting its foes.

Late on Wednesday, a court ruled that Khan's march would be permitted, as long as nothing illegal was done, and Sharif's interior minister said the government would respect that ruling.

Allowing Khan's march demonstrated that the government tolerated peaceful protests and at least sometimes obeyed the courts, which are rapidly emerging as Pakistan's third power centre after the military and the fledgling civilian government.

At Khan's home, protesters were exuberant despite a huge traffic jam as they tried to set off on the 370-km journey from Lahore to Islamabad, said Asad Umar, a member of parliament from Khan's party.

Khan travelled in a modified, bulletproof shipping container with windows. Many of his supporters carried sleeping mats and food, determined to camp on Islamabad streets until their demands were met - including a demand for Sharif to resign.

"I was treated at his cancer hospital free of cost," said 50-year-old housewife Aasia Khan, referring to a charitable hospital that Khan set up in memory of his mother. "I owe him a lot and will support him until I die."

Khan's political ambitions were for years dismissed but he built up support, in particular among students. The one-time playboy cricket star developed a reputation as a conservative maverick, who, among other things, questioned Pakistan's close ties with the United States.

He won 34 seats in the 342-seat lower house of parliament in the last election. Sharif's party won 190 seats.

Khan is protesting alleged irregularities in the polls, which marked the first transfer of power from one elected government to another in coup-prone Pakistan's history.

As Khan's supporters prepared to march, Qadri's supporters, who had planned to join them, were isolated and blockaded in the area around his home by shipping containers placed across roads by the authorities. Those inside said food and water were running low and telephone services in the area were suspended.

The fiery cleric had vowed to overthrow the government by the end of this month. His supporters, many of them drawn from his network of Islamic schools and charities, have been involved in several deadly clashes with police.

One of Qadri's main complaints is that the killing of supporters by police is not being properly investigated.