CAIRO, Egypt - They're the most feared men on the streets of Iran.

The pro-government Basij militia has held back its full fury during this week's demonstrations. But witnesses say the force has unleashed its violence in shadowy nighttime raids, attacking suspected opposition sympathizers with axes, daggers, sticks, and other crude weapons.

At least once, the militiamen opened fire on a crowd of strone-throwing protesters. State media said seven were killed.

If Iran's supreme leader, Ayatollah Ali Khamenei, authorizes a crackdown on protesters calling for a new presidential election, as he warned yesterday, the Basij will almost certainly be out in force.

Formed during the 1979 Islamic Revolution, the Basij (buh-SEEJ) became one of Iran's most zealous forces in the Iran-Iraq war of the 1980s, often leading charges through minefields.

The group, controlled by the elite Revolutionary Guard, also was unleashed on dissidents in the 1990s, when teenagers and young men in plainclothes beat protesting students with batons. It is an intimidation tactic that opposition supporters say has been revived during this week's outpouring of antigovernment demonstrations.

"The Basij began as cannon fodder for the Revolutionary Guard during the war with Iraq," said Meir Javedanfar, an Iranian-born independent analyst living in Israel. "Now, they are there to do the dirty work for them: breaking up parties, hassling women about their hijab [head covering], and much more violent acts."

The Basij has leaders based in mosques in every Iranian village and city, giving it the widest security network in the country, said Mehdi Khalaji, a senior fellow with the Washington Institute for Near East Policy and a specialist on Iran.

A million or more

The Iranian government says the Basij has five million members in total, but Khalaji said active members number about one million.

The Revolutionary Guard, a military force that answers to Iran's supreme leader, is considered a strong supporter of President Mahmoud Ahmadinejad. The Basij was used to mobilize support for him in the 2005 election as well as in last week's vote, Khalaji said.

In addition to their salaries, militia members - known as Basijis - get incentives such as easy entrance to universities and licenses and loans for businesses.

The most senior members are issued guns. But the majority use sticks, pepper spray, and other crude weapons.

Infiltrating

"They carry guns, batons, and they are driving motorcycles," Khalaji said. "With the motorcycles they go suddenly, they start to drive into the crowd with high speed."

Some Basijis shave their beards and wear jeans to blend in with opposition supporters, infiltrating a crowd and then attacking, he said.

Amateur videos and photos from Iran posted online in recent days have shown what appear to be attacks on people and property carried out by young men in ordinary clothing. The images cannot be authenticated because of government restrictions on the media and phone and Internet communication in and out of Iran.

Khamenei's personal bodyguards, who protect his home and office, control Tehran's Basij force, Khalaji said, and his stern warning yesterday of a crackdown if protests continue was an unambiguous threat to send the militiamen into the streets.