Hundreds of grad rockets and mortar rounds landed on frontline areas west of the city throughout the day, with the rebels reporting at least two dead and 25 wounded.
The shelling came as Mr Zuma prepared to deliver a message from both Nato and Moscow demanding that the Libyan leader step down.
This demand was agreed by world leaders meeting at last week's G8 summit in France, with Russia joining Britain, France, Italy and the United States as saying that Col Gaddafi's resignation was now the only route to peace.
It was a message echoed yesterday by Abdul Musfafa Jalil, president of the rebel Transitional National Council. "The basis of any consideration for the resolution of the Libyan crisis, is the removal of the main reason for this crisis, Col Gaddafi," he said.
Nato is expected to temporarily lift its no-fly zone to allow Mr Zuma's presidential Boeing 737 to cross Libyan air space for the meeting.
How much room for manoeuvre Mr Zuma has is unclear. Gaddafi has already offered a ceasefire to the rebels who have rejected it, saying the regime must first end torture and repression in territory still under government control.
Now, rebel sources say, only Gaddafi's departure will suffice. There is anxiety both in the rebel corridors of power and foreign capitals that Col Gaddafi will try to spin out talks with Mr Zuma, hoping to get a ceasefire that will allow him time to re-organise his battered forces.
Tripoli itself gave no sign of whether it would accept the G8 demand for Col Gaddafi to step down. Libya's foreign minister Khaled Kaaim said: "We are an African country. Any initiative outside the AU (African Union) framework will be rejected."
Although Mr Zuma is making a private visit, he is part of the Libyan affairs committee of the AU, and the organisation would be likely to welcome a mediation role if it were offered. The more substantial problem is that the key world powers have agreed that there is little to mediate about beyond Col Gaddafi's immediate departure.
Part of Mr Zuma's message will be that, should the Libyian leader fail to agree terms, Nato is ready to begin operations with British Apache and French Tiger attack helicopters. which are already deployed on warships in the Mediterranean.
The helicopters will bring pinpoint firepower to an air war which has failed to break a month-long stalemate. Rebel forces hold a front line south-west of their capital, Benghazi, and have now pushed Gaddafi's forces out of Misrata city.
Yet the lack of heavy weapons has hampered the ability of the rebels to take the war to the enemy, even with continuing air strikes from Nato jets.