Explosions are rocking Damascus as Syrian troops clash with rebels in some of the heaviest fighting yet in the capital in the 15-month uprising against President Bashar Assad.
Troops have also unleashed a heavy assault to retake a rebel-held neighbourhood in a central flashpoint city, blasting it with heavy bombardment.
Also on Friday, UN observers entered a remote farming area where a massacre was reported this week, an activist said, a day after they were blocked from reaching it by troops and local residents and fired upon.
The fighting in Damascus erupted in the restive district of Kfar Souseh, where the night before, armed rebels took part in a large anti-government rally in the same district, witnesses said - a rare and bold public appearance by the fighters in the capital.
Friday's fighting began when the fighters attacked a government checkpoint in the morning, according to Rami Abdul-Rahman, of the Britain-based Syrian Observatory for Human Rights.
A witness said explosions could be heard throughout central Damascus and that smoke could be seen rising from the area.
In the central city of Homs, one of the main battlegrounds of the uprising, regime troops were trying to advance into the opposition-held district of Khaldiyeh from three sides, battling with armed rebels trying to stop them, said Tarek Badrakhan, an activist based in the neighbourhood speaking via Skype.
"This is the worst shelling we've had since the start of the revolution," he said. A shell could be heard exploding in the background as he spoke.
Shells were hitting the neighbourhood at a rate of five to 10 a minute, said the Britain-based Syrian Observatory for Human Rights. "It seems they are trying to enter it today," it said.