Explosions and heavy gunfire stopped Islamist politicians from entering a besieged Islamabad mosque yesterday, on a mission to persuade a radical cleric to send out children among his hundreds of militant followers.

The five-member delegation of religious conservatives blamed security forces for opening fire, when the cleric had already given them an all-clear to enter the Red Mosque, or Lal Masjid.

Hundreds of troops have besieged the fortified compound housing the mosque and Jamia Hafsa, a girls' madrasa, or school, since Tuesday when months of tension erupted into clashes.

There were unconfirmed accounts of the mosque's defenders burying more bodies yesterday, but so far the death toll is 20 following the death of a soldier in gunfire in the morning.

President Pervez Musharraf struck an uncompromising tone. Mosque cleric Abdul Rashid Ghazi, the leader of a Taliban-style movement, has said he would rather choose "martyrdom", and has rejected government accusations that he is holding women and children as human shields. Yesterday, army soldiers were deployed round the mosque, replacing paramilitary troops who have led the siege.

Water, gas and power to the mosque have been cut and food was said to be getting scarce.

There was a series of explosions in late afternoon, while there had been several exchanges of fire since early morning.

About 1,200 students left the mosque after the clashes began but only about 20 came out on Friday, among them a boy who said older students were forcing young ones to stay.

Officials say they don't know how many are left inside, though they put the number of hard-core militants at 50 to 60. Ghazi has said there are 1,900 students in the compound, while his brother, who was captured trying to escape in a burqa on Wednesday, put the number at 850, including 600 females. Authorities say they have blasted holes in the compound's walls to enable people to flee. Security forces have also occupied another city madrasa affiliated with the Lal Masjid.

Tensions began rising in January when students launched an anti-vice campaign to impose strict Islamic law. They kidnapped people they accused of prostitution, intimidated shopkeepers selling Western videos, abducted police and threatened suicide attacks if they were suppressed.

Moderate politicians and the media had urged Musharraf to crack down on the Red Mosque radicals far earlier, and despite the bloodshed newspaper editorials have shown broad support for the decision to finally use force.

The Red Mosque movement is symptomatic of the religious extremism seeping into Pakistani cities from tribal border areas. Musharraf has urged security agencies to allow time to get children out.

On Friday, gunmen fired from a roof-top under the flight path of Islamabad's military airport as Musharraf was flying off to inspect flood damage in the south.

Officials said privately the shooter meant to target Musharraf's aircraft, though the attack appeared amateurish.