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By Zeeshan Haider

ISLAMABAD, July 4 (Reuters) - The head of a radical Pakistani mosque at the centre of a bloody stand-off with security forces was arrested on Wednesday while trying to escape clad in a woman's burqa, officials said.

The arrest of Abdul Aziz, chief cleric of Islamabad's Red Mosque, was a major coup for the government.

But two earlier bomb attacks on security forces in another part of the country that killed 12 people raised fears militant supporters of the mosque were hitting back.

Up to 1,200 radical Muslim students based at the besieged mosque took up an offer of safe passage and 5,000 rupees ($85) and surrendered, a day after clashes outside the mosque.

Aziz tried to slip out among women from a mosque school, who all wear black, all-enveloping burqas.

"He was trying to escape wearing a burqa. He was caught at the checkpoint where women leaving the mosque have to register as some policemen found his appearance suspicious," said deputy city administrator Chaudhry Mohammad Ali.

Sixteen people have been killed in the violence that erupted after a months-long stand-off between the authorities and a Taliban-style movement based at the Lal Masjid, or the Red Mosque, less than two kilometres (a mile) from parliament and the capital's diplomatic enclave.

Aziz runs the mosque with his brother, Abdul Rashid Ghazi, who was believed to be still inside, along with many militant supporters who were defying government ultimatums to surrender.

Hundreds of police and soldiers, backed by armoured personnel carriers and with orders to shoot armed resisters on sight, sealed off the mosque and imposed an indefinite curfew in the neighbourhood after Tuesday's clashes.

Television showed Aziz after his capture with the top of his burqa pulled off. A balding heavy-set man with a grey bread, police bundled him into a car. Aziz's wife, Umm-e-Hassan, was also caught while trying to slip away.

The government said earlier it would not negotiate with the brothers. "They have no option but to surrender," Interior Ministry spokesman Javed Iqbal Cheema told reporters.

"WE"LL FIGHT"

Liberal politicians have for months pressed President Pervez Musharraf to crack down on the clerics, who had threatened suicide attacks if force was used against them.

Some of Musharraf's critics have suggested the government initially saw the trouble as a welcome distraction from a judicial crisis that erupted when Musharraf suspended the country's top judge in March.

But heavy loss of life in any assault would be very damaging for Musharraf in the run-up to elections this year.

No one knows how many students, who range in age from teenagers to people in their 30s, remain in the mosque. Officials gave estimates from several hundred up to 5,000.

Former intelligence chief Asad Durrani said the authorities would want to use minimum force and the surrenders and the capture of Aziz might reduce the likelihood of an assault.

But a security official said a small, hard core was unlikely to give up. A spokesman for the mosque, Abdul Qayyhum, was defiant: "Our moral is high and we'll fight."

Security forces fired teargas into the compound in the afternoon and exchanged intermittent fire with mosque gunmen. Anti-terrorism police moved into the area as darkness fell and several helicopters flew overhead.

A suicide bomber killed six soldiers and two children in North West Frontier Province and a roadside bomb aimed at police killed four civilians in another part of the province.

Police chief Sharif Virk said no one claimed responsibility for the attack on his men but a cleric, Fazalullah, with links to the Islamabad mosque, was active in the region.

The Lal Masjid movement is part of a phenomenon known as "Talibanisation", or the seeping of militancy from remote tribal regions on the Afghan border into central areas.

The mosque has a long history of support for militancy but the latest trouble began in January when students occupied a library to protest against the destruction of mosques illegally built on state land. They later kidnapped women they said were involved in prostitution and abducted police. (Additional reporting by Kamran Haider)

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