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 street 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.

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

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

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

The group, which is 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's an intimidation tactic opposition supporters say has been revived during this week's outpouring of anti-government protest.

"The Basij began as cannon fodder for the Revolutionary Guard during the war with Iraq. 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," said Meir Javedanfar, an Iranian-born independent analyst living in Israel.

The Basij has leaders based in mosques in every village and city throughout Iran, 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 in Iranian politics.

The Iranian government says there are 5 million members in total, but Khalaji told The Associated Press on Friday that active members number around 1 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 during 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. "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. They beat people with electric batons."

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 photographs from Iran posted online in recent days have shown what appear to be attacks on people and property in cities around Iran carried out by young men wearing ordinary clothing. The images cannot be authenticated because of Iranian government restrictions on the media and telephone and Internet communication in and out of the country.

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

Members of the Basij and the Revolutionary Guard were on the streets of Tehran after midday prayers Friday, though not in overwhelming numbers.

So far, the Basij has refrained from widespread attacks on demonstrators. But witnesses say the militiamen took part in a police raid on Tehran University dormitories on Sunday night after students hurled stones, bricks and firebombs at police — one of the few violent episodes during this week's rallies.

Basij members used axes, sticks and daggers to ransack student rooms and smash computers and furniture, wounding many students, according to witnesses.

A day later, students attacked a compound used by the Basij and tried to set it on fire. Gunmen on the roof fired on the crowd and killed seven people, according to state media.

Amateur videos that appear to be from that clash showed men carrying away the wounded on streets spattered with blood as fires burned in the distance and gunfire crackled.