Syrian authorities raided restive areas in capital Damascus and central Homs province on Friday to track down armed groups, as a number of explosive devices went off in several Syrian cities, causing injuries and property losses.

Acting on a tip, authorities raided a number of "terrorist" hideouts in the Damascus' suburb of Douma and its outskirts, killing three terrorists, and either injuring or arresting others, said state-run SANA news agency, quoting an official source.

SANA also reported that Syrian authorities raided a hideout of armed groups in central Qusair town, killing unspecified number of terrorists and injuring several others, while a pro-government website page said that armed groups had set the municipal building in Qusair ablaze, and fierce clashes are still underway.

In the meantime, five members of law-enforcement forces, one of them a first lieutenant, were injured when an explosive device was detonated near al-Jawwash roundabout in Hama province on Friday, said SANA, adding the injured were rushed to Hama National Hospital.

Also on Friday in central Midan neighborhood in Damascus, nine civilians and members of the military engineering units were injured in the blast of an improvised explosive device planted by an "armed terrorist group" in al-Qa'a region behind al-Majed Mosque in al-Midan, according to SANA.

Another explosive device was detonated overnight Friday at Bab Sharqi area in the heart of the old Damascus, local media said, giving no further details.

Sound of explosions could be heard reverberating across Damascus after midnight Friday.

In Aleppo province, two civilians were injured in a blast of an explosive device that was planted in a waste container.

Also in Aleppo, six civilians and three law-enforcement members were injured when an explosive device, planted by an armed terrorist group near Saad Bin Abi Wakkas Mosque in Salaheddin neighborhood went off, causing material damage.

On the opposition side, the activists' network, Local Coordination Committees (LCC), said that more than 40 people were killed across Syria on Friday. It said one man was killed and others injured in the army shelling on the suburb of Douma. It added the army also shelled the Damascus' suburbs of Harasta and Deir al-Asafir.

The LCC said overnight protests erupted in a number of Syrian cities Friday.

Another activist group, the Britain-based Syrian Observatory for Human Rights, put the death toll of Friday's violence at 30. The Observatory sent an appeal to humanitarian organizations to send relief aid to unrest-affected people in several neighborhoods of Homs due to "army bombardment".

The activists' account couldn't be independently verified.

Earlier in the day, head of the UN supervision mission to Syria Maj-Gen Robert Mood observed that violence has escalated over the past 10 days in Syria from all parties, adding that such violence hampers his observation mission.