A suicide bomber has killed nine people and injured dozens in the Syrian capital, Damascus, according to state media.
The developments on Friday came a day after Ban Ki-moon, the UN secretary-general, said the Syrian government was "in contravention" of an internationally agreed peace plan by keeping troops and heavy weapons in cities.
The SANA news agency said the victims of Friday's blast in al-Midan district included civilians and law enforcement personnel.
Opposition activists said the blast struck opposite the Zain al-Abideen mosque, which was under heavy security for Friday prayers, and where regular protests against President Bashar al-Assad have been held in the last year.
Syrian TV aired footage of white smoke billowing from under a bridge as people streamed out of a mosque. The streets were stained with blood. There was no immediate claim of responsibility.
A resident who spoke to security officials at the scene told the Reuters news agency that a man had approached soldiers near the mosque and detonated a bomb belt when challenged.
In January, an explosion in the same neighbourhood killed at least 26 people, according to state media.
A loud blast was also heard in the capital's al-Sinaa district on Friday, near a garage used by government buses and shabiha, pro-Assad militiamen tasked with preventing demonstrations.
A third explosion, in the Adawi area, near the agricultural department of Damascus University, was confirmed both by pro-government media and opposition activists.
'Action picking up'
Central Damascus has so far been spared much of the violence since the uprising began in March last year. But on Wednesday, a bomb went off in the Marjeh neighbourhood in the centre of the capital.
"The action is picking up and it seems the [opposition fighters] and Assad's forces are starting to battle it out in Damascus as well," Mar Ram, an activist in al-Midan, told Reuters.
In-depth coverage of escalating violence across Syria
The government blames armed groups for the recent blasts, but some opposition leaders accuse the government of carrying out the attacks as a way to tarnish the uprising.
On Friday, activists reported that large protests were held in the northern city of Aleppo and many towns and villages, including in the central region of Hama and the northern province of Idlib.
The Syrian Observatory for Human Rights said three people, including a child, were killed as government forces opened fire to disperse protests, in Damascus province, Aleppo, and Deir al-Zor.
Three members of the security forces and a defector were also killed in clashes across the country, the group said.
Assad's government has agreed a troop withdrawal from cities as part of a six-point peace plan for Syria agreed with Kofi Annan, UN-Arab League envoy.
The truce, which officially started on April 12 but has been violated by both government forces and opposition fighters, is to be monitored by 300 UN observers due to arrive in Syria in coming weeks.
A small advance team is already on the ground, with monitors deployed in the flashpoints cities of Homs, Deraa, Hama and Idlib, as well as in Damascus.
The UN on Friday appointed Norway's Major General Robert Mood to head the monitoring force.
Mood has held several senior military posts internationally, including in UN peacekeeping operations in Lebanon and heading the UN observer mission to the Middle East.
Opposition activists say more than 300 people have been killed since the first monitors arrived.
The government has reported hundreds of violations of the truce by opposition fighters launching attacks on security forces.
NATO chief Anders Fogh Rasmussen said on Friday that Syria was not respecting the Annan peace plan, but added: "We have no intention to intervene in Syria. We believe the right way forward is to ensure a political, peaceful solution."
Meanwhile, the UN chief said he was "gravely alarmed" by the continued killings. He said he hopes the speedy deployment of all the truce monitors "will change the dynamics on the ground".'
A UN statement also said Ban "remains deeply troubled by the continued presence of heavy weapons, military equipment and army personnel in population centres, as reported by United Nations military observers".