Syrian rebels attacked key military targets and overran two police stations in Aleppo, killing 40 officers, a watchdog said, as the pivotal battle for the commercial capital raged on Tuesday.
Clashes between the rebels and loyalists of President Bashar al-Assad were also reported in the capital Damascus, the eastern city of Deir Ezzor and Daraa in the south, cradle of the more than 16-month uprising.
The Syrian Observatory for Human Rights said Aleppo was on Tuesday rocked by the fiercest fighting of a military offensive on rebels in the city, which came after the government had warned of a looming "mother of all battles."
Rebels used rocket-propelled grenades in pre-dawn attacks on a military court, an air force intelligence headquarters and a branch of the ruling Baath Party in Aleppo, said the Observatory's Rami Abdel Rahman.
Later, "hundreds of rebels attacked the police stations in Salhin and Bab al-Nayrab (neighbourhoods) and at least 40 policemen were killed during the fighting, which lasted for hours," Abdel Rahman told AFP.
The police chief was among those killed at the Salhin station in the south of the city, while three vehicles were destroyed, he added.
The attacks came as the UN observer mission said government forces were using helicopters, tanks and artillery to fight the rebels, while appealing for both sides to protect civilians in the city of 2.7 million people.
Through the night, government troops had shelled the neighbourhoods of Salaheddin, Marjeh, Firdoss, Al-Mashhad, Sakhur, Al-Shaar and Ansari, before the army and rebels clashed at dawn in Al-Meesr and Al-Adaa.
A security official in Damascus told AFP on Monday that the army had regained some of Salaheddin but it was facing "a very strong resistance." The rebels, however, denied that the army had advanced even "one metre" (yard).
"The fierce fighting in Aleppo shows how crucial this city is for a regime that does not want a Benghazi in Syria," said Abdel Rahman, referring to the coastal city secured by Libyan rebels as a base in their fight to bring down strongman Moamer Kadhafi.
Gunmen from loyalist Arab tribes in Aleppo, including the al-Berri family, had joined the fray and were fighting alongside the army.
"All of this links back to calls by Syrian media and talking heads on some Lebanese satellite stations that loyal Syrian citizens should take up arms and fight with the regime troops," Abdel Rahman added.
Rebels on Monday seized the strategic Anadan checkpoint, some five kilometres (3.8 miles) northwest of Aleppo, securing a direct route to the Turkish border.
"During the next few hours, the impact of rebel control over this checkpoint will be proven by the amount of supplies brought to Aleppo," said Abdel Rahman.