Libyan rebels battled their way towards Tripoli on Sunday to help fighters inside the city who rose up overnight declaring a final showdown with Muammar Gaddafi.

The Libyan leader dismissed the rebels, fighting since February to topple him, as 'rats' and said he would not yield.

In a coordinated revolt that rebel cells had been secretly preparing for months, shooting started on Saturday night across Tripoli moments after Muslim clerics, using the louspeakers on mosque minarets, called people on to the streets.

The fighting inside Tripoli, combined with rebel advances to the outskirts of the city, appeared to signal the decisive phase in a six month conflict that has become the bloodiest of the 'Arab Spring' uprisings and embroiled Nato powers.

But Gaddafi's fall is far from certain. His security forces did not buckle, the rebels appeared to control only a few neighbourhoods of Tripoli and the city is much bigger than anything the mosly amateur anti-Gaddafi fighters, with their scavenged weapons and mismatched uniforms, have ever tackled.

If the Libyan leader is forced from power, there are question marks over whether the opposition can restore stability in this oil exporting country. The rebels' own ranks have been wracked by disputes and rivalry.

Rebels said that after a night of heavy fighting, they controlled a handful of city neighbourhoods. But whether they hold on could depend on the speed with which the rebels elsewhere reach Tripoli.

'The rebels may have risen too early in Tripoli and the result could be  a lot of messy fighting,' said Oliver Miles, a former British ambassador to Libya. 'The regime may not have collapsed in the city to quite the extent they think it has.'     

The closest front line was to the west of the capital, along a highway that traces the edge of the Mediterranean Sea.

Rebel fighters returning from the front line said they they had taken the town of Jaddaim and that they were now about 20 km from Tripoli and approaching the city's outlying western suburb of Janzour.

A Reuters reporter near the front said he could hear shells landing, and could see columns of smoke. Ambulances rushed back from the front to a hospital in the nearby town of Zawiyah.

In Jaddaim, fighters were celebrating the advance, shouting 'Allahu Akbar!' or 'God is greatest!.'     

In Benghazi, the eastern Libyan city where the anti-Gaddafi revolt started and where the rebels have their main stronghold, a senior official said everything was going according to plan.

'Our revolutionaries are controlling several neighbourhoods and others are coming in from outside the city to join their brothers at this time,' Abdel Hafiz Ghoga, vice-chairman of the rebel National Transition Council, told Reuters.           

A spokesman for Gaddafi, in a briefing for foreign reporters, underlined the message of defiance. The armed units defending Tripoli from the rebels 'wholeheartedly believe that if this city is captured the blood will run everywhere so they may as well fight to the end,' said the spokesman, Moussa Ibrahim.

'We hold Mr Obama, Mr Cameron and Mr Sarkozy morally responsible for every single unnecessary death that takes place in this country,' he said, referring to the leaders of the United States, Britain and France.

A diplomatic source in Paris, where the government has closely backed the rebels, said underground rebel cells in the capital had been following detailed plans drawn up months ago and had been waiting for a signal to act.      

That signal was 'iftar' -- the moment when Muslims observing the holy months of Ramadan break their daily fast. It was at this moment that imams started broadcasting their message from the moques, residents said.

But the overnight fighting inside the city, while fierce, was not decisive. Rebels said they controlled all or parts of the Tajourah, Fashloom and Souk al-Jumaa neighourhoods but there was no city-wide rebellion.

In Tripoli on Sunday, the two sides appeared to be jockeying for control of rooftop terraces where they could place firing positions, possibly in preparation for a new burst of fighting after nightfall. - Reuters