* Tens of thousands aim to occupy Islamabad's main street
* They say government is corrupt, should step down
* Unrest raises fears for civilian rule in coup-prone state (Updates march numbers, quotes, plan to occupy capital's main street)
By Mubasher Bukhari
LAHORE, Pakistan, Aug 14 (Reuters) - Tens of thousands of anti-government protesters began a march to the Pakistani capital Islamabad on Thursday, raising fears for political stability and civilian rule in the nuclear-armed South Asian country.
Two protest groups - one led by cricketer-turned-opposition politician Imran Khan, the other by activist cleric Tahir ul-Qadri - are heading to the capital from the eastern city of Lahore. They say the government is corrupt and should step down.
Both marches were initially banned, then allowed to go ahead at the last minute. The protesters caused huge traffic jams, and by evening the leaders and most marchers had not left Lahore.
Reuters reporters in Lahore and Peshawar said tens of thousands of people were congregating for the marches.
Khan and Qadri are not officially allied though both are enemies of the government of Prime Minister Nawaz Sharif, whose party swept an election last year. It was the first transfer of power from one elected government to another in the history of coup-prone Pakistan.
Qadri is a Muslim preacher turned political activist who lives most of the time in Canada. His supporters, many from his network of Islamic schools and charities, have been involved in several deadly clashes with police.
In Islamabad, security was tight. Main roads and key areas, including many embassies, were blocked by riot police and shipping containers. But the country's interior minister said the marchers would be allowed to enter.
Qadri's spokesman said they planned to occupy Jinnah Avenue, the main street in the capital near many embassies and top government offices.
Qadri has said he plans to force out Sharif and his government by the end of the month. "There will be a sit-in. They will stay there until their demands are met and (Sharif) steps down," Qadri told Reuters.
The cleric's calls for revolution are appealing to poor Pakistanis struggling with high unemployment, daily power cuts and inflation. So are the promises he makes.
"Every homeless person will be provided housing; every unemployed person will be given a job; low-paid people will be provided with daily necessities," Qadri said on Thursday.
One of his main complaints is that violence against his supporters by police is not being properly investigated. About 2,000 of his supporters have been arrested, police say.
The political confrontation has revived concern about the central issue in Pakistani politics: competition for power between the military and civilian leaders.
Some officials have accused elements within the powerful military of orchestrating the protests to weaken the civilian government. The military insists it does not meddle in politics.
Most analysts doubt the military wants a coup, but a perception is widespread that it could exploit the protests to pressure the civilian government.
Despite those perceptions, Sharif is relying on the military for security in the face of the challenges. Recently, policies that the armed forces object to, such as the treason trial of former military leader Pervez Musharraf, have ground to a halt.
Imran Khan said he was cheated in the general election in May last year and wants a proper investigation into his complaints.
His supporters were exuberant as they set off on the 370-km (230-mile) journey to Islamabad on Thursday, an Independence Day holiday in Pakistan.
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 dismissed for years, but he built up support, particularly among students. The one-time playboy cricket star developed a reputation as a conservative maverick and reformist.
On Thursday, he challenged Sharif over tax evasion and the country's dependence on foreign donors. Most wealthy Pakistanis pay almost no tax, but the nation receives billions in aid.
"The begging bowl will break only when you start paying taxes yourself," he said in a speech.
Khan won 34 seats in the 342-seat lower house of parliament in the last election. Sharif's party won 190 seats. Qadri did not contest. (Additional reporting by Katharine Houreld, Mehreen Zahra-Malik and Syed Raza Hassan in Islamabad and Asim Tanveer in Multan; Writing by Katharine Houreld; Editing by Mark Heinrich)