Army units that are loyal to Muammar Gaddafi have entered the city of Benghazi, the main stronghold of Libyan rebels east of Tripoli.
According to the Qatari satellite TV channel Al-Jazeera, the attack was preceded by intensive artillery shelling.
No casualties in killed or wounded have been reported so far.
According to the France Presse news agency, a military plane has been shot down over the city.
Meanwhile the Libyan authorities, who said they were ending military action against the rebels after the UN Security Council adopted a restrictive resolution on Friday, claimed later that Government troops came under attack at the approaches to Benghazi.
Analysts believe that today’s developments will prompt the West’s speedy decision to deliver strikes on Libya.