CAIRO: Supporters of President Hosni Mubarak opened fire on protesters in Cairo's Tahrir Square on Thursday, killing at least five, in a fresh spike in violence over an unprecedented challenge to his 30-year-old rule.

The opposition responded by renewing demands that he quit.

Thousands of dissidents barricaded themselves into the central Cairo square, vowing to remain until Mubarak goes. For the first time, troops deployed to create a buffer zone of 80 metres or so between them and pro-Mubarak groups.

The Muslim Brotherhood, a formally banned mass movement seen as the best organised opposition, issued a statement calling for a national unity government to replace Mubarak. The group, whose potential rise to power troubles Egypt's Western allies, has so far taken a backseat in the protest movement.

In the overnight fighting, machinegun fire echoed for more than an hour across the central square where protesters are unsatisfied by Mubarak's pledge to step down only in September.

By daylight there was a lull. Troops with tanks continued to look on but took more steps than on Wednesday to separate the two sides as pro-Mubarak supporters were seen moving again toward the square with knives and sticks.

"One way or another we will bring Mubarak down," some protesters chanted in the early morning. "We will not give up, we will not sell out," others shouted.

In a statement on Al Jazeera, the Brotherhood said: "We demand that this regime is overthrown and we demand the formation of a national unity government for all the factions."

Mubarak has offered to talk, and state television said on Thursday that his newly appointed vice-president, Omar Suleiman, had spoken with unspecified groups.

A spokesman for the new government, which Mubarak named this week in a vain bid to appease protesters, denied it was involved in the violence and said it would investigate.

Egyptian Health Minister Ahmed Samih Farid told state television five had died and 836 were wounded in fighting which first erupted on Wednesday. He said most of the casualties were due to stone throwing and attacks with metal rods and sticks.

The army, for 60 years the power behind Egypt's presidents, has given protesters heart by calling their demands legitimate. But it has also largely not intervened to protect them.

The government spokesman said on Thursday that troops had not taken part to avoid showing favour to either side.

Hundreds of those protesting in the square wore bandages and other signs of being hurt. Mohamed al-Samadi, a doctor who had been treating people complained the troops were not helping.

"When we come here, they search us for weapons, and then they let armed thugs come and attack us," he said. "We refuse to go. We can't let Mubarak stay eight months."

Gunfire

The firing began around 4 a.m. while hundreds of anti-government protesters camped out in the square.

With many protesters blaming the government for instigating the crackdown on the previously largely peaceful demonstrations, the United States has renewed its appeal to Mubarak to take steps toward democratic elections at once.

A senior US official also said on Wednesday it was clear that "somebody loyal to Mubarak has unleashed these guys to try to intimidate the protesters."

Washington supplies the Egyptian army — which has ruled Egypt since toppling the monarchy in 1952 — annually with about $1.3 billion in aid.

But its options for leaning harder on Egypt to end the violence and begin a transfer of power are limited.

Egypt, which signed a peace treaty with Israel in 1979, justified the emergency rule which kept Mubarak in power as needed to curb religious extremists and Washington is looking for a way forward which does not encourage even greater instability.

After Mubarak announced on Tuesday that he would stay in office until September and then step down, President Barack Obama telephoned him and said that change "must begin now." He stopped short of calling him to quit immediately.

On Wednesday chances of a peaceful resolution of the crisis receded when supporters of Mubarak, throwing petrol bombs, wielding sticks and charging on camels and horses, attacked protesters in Tahrir Square.

Reacting to the tumult in Egypt, White House spokesman Robert Gibbs said on Wednesday that, "If any of the violence is instigated by the government it should stop immediately."

An estimated 150 people have been killed so far and there have been protests across the country. United Nations human rights chief Navi Pillay said up to 300 people may have died.

Opposition rejects talks

Egyptian Vice President Omar Suleiman on Wednesday urged the 2,000 demonstrators in Tahrir Square to leave and observe a curfew to restore calm. He said the start of dialogue with the reformists and opposition depended on an end to street protests.

But the opposition, which includes the Muslim Brotherhood and Nobel peace laureate Mohamed ElBaradei, have said they will not open talks until Mubarak quits.

"What happened yesterday (Wednesday) made us more determined to remove President Mubarak," the protest movement Kefaya, or Enough, said.

"There will be no negotiations with any member of Mubarak's regime after what happened yesterday and what is still happening in Tahrir Square," a spokesman told al Jazeera.

An Egyptian Foreign Ministry statement on Wednesday rejected US and European calls for the transition to start immediately, saying they aimed to "incite the internal situation" in Egypt.

Oil prices have climbed on fears the unrest could spread to other authoritarian Arab states including oil giant Saudi Arabia or interfere with oil supplies from the Red Sea to the Mediterranean through the Suez Canal. Brent crude surpassed $103 a barrel on Thursday.

On Thursday, thousands of anti-government protesters gathered in the Yemeni capital Sanaa demanding a change in government and saying President Ali Abdullah Saleh's offer on Wednesday to step down in 2013 was not enough.

Some analysts suggested the violence could encourage a backlash against Mubarak both internationally and among those Egyptians who had accepted his pledge to step down in September.

Along with the United States, France, Germany and Britain have also urged a speedy transition.

But others feared he could either cling on to power, or hand over to a military ruler without allowing truly free and fair democratic elections.

"Mubarak fooled us all," one person in Tahrir Square shouted through a loudspeaker.