"Allahu Akbar, the army and the people are hand in hand," chanted protesters barricaded in Tahrir Square, where 10,000 people joined the hundreds who had camped overnight.
Mubarak loyalists fled when the tank turned its turret towards them, then regrouped nearby and resumed throwing stones.
Gunfire was heard in the square, the epicentre of rallies seeking to end Mubarak's three decades of authoritarian rule.
Doctors at a makeshift clinic operating out of a nearby mosque said 10 people had been killed during the clashes.
"An hour an a half ago, two people were rushed to me with gunshot wounds to the head. They were gasping and died. A third case followed, which was very severe, but that case was taken to hospital by ambulance," said one doctor, Yasser Tibi.
The health minister earlier said more than 800 were wounded.
Egypt's new Prime Minister Ahmed Shafiq apologised for the violence in Cairo, which protesters demanding Mubarak's resignation say was instigated by his loyalists.
As evening fell, the square erupted with renewed activity and protesters chanted "The people demand the fall of the regime" as Vice President Omar Suleiman appeared on television to say Mubarak would not put himself forward for the presidency again.
Al Arabiya television reported separately that Mubarak supporters had stormed hotels, chasing foreign journalists.
Amnesty International said two of its staff were held and Human Rights Watch said its researcher was picked up in a raid and taken to an unknown location. The BBC said a camera and a laptop had been seized from its staff in Cairo.
On Monday the army emboldened protesters by endorsing their demands as legitimate and pledging not to open fire on them. But until Thursday the soldiers had not really intervened.
In Egypt's second largest city of Alexandria, thousands protested with anti-Mubarak banners, one reading: "In all languages of the world, we tell you: go out, Mubarak."
In northeastern Egypt, around 4,000 people marched in Suez calling for Mubarak to step down, while in Ismailia a crowd of 2,000 held a similar demonstration.
In Cairo, protesters lined up stones to fight off attacks by pro-Mubarak groups.
"We are using these stones as a means of defence. Yesterday they attacked us with molotov cocktails (petrol bombs) and all we have to protect ourselves with is stones," said Ali Kassem.
Violence
A Reuters journalist saw protesters overpower someone they said was an undercover member of the security services.
Over a loudspeaker a voice urged: "Don't beat him. Hand him to us and the organising committee and we will hand him over to the army. The international media is watching us and saying we are peaceful people."
Some protesters say the pro-Mubarak supporters are being paid by Mubarak's National Democratic Party (NDP).
Mobile phone operator Vodafone accused the Egyptian authorities of using its network to send pro-government text messages to subscribers, without clear attribution. One message sent on Feb. 2 seen by Reuters announced the location and timing of a pro-Mubarak rally.
"If they don't want the president to go, okay, but let's see whose voice is louder ... what I saw last night made me want to stay longer, it strengthened my resolve," said Ilham Farouk, a 27-year-old pharmacist in a full face veil.
Egypt's health minister said earlier six people were killed and 836 wounded in the overnight violence. Mubarak loyalists opened fire and threw stones and petrol bombs. Protesters barricaded themselves in the square and hurled stones back.
"Through the night we were getting dozens of wounded every 15 minutes. We had casualties all over the place," said Mohamed Abdel Hamid, a doctor.
"Thugs surrounding us tried to attack more of us but we managed, thankfully, to block their advance."
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