Syrian authorities strengthened security measures in Damascus as the opposition marks the second anniversary of the country's uprising.
Protests were held in several towns on Friday, with some rebels fighting to topple President Bashar al-Assad calling for stepped-up attacks on the government to mark the anniversary.
The developments on the ground came as the European Union said it would seek "a common position" at talks in Dublin next week on whether to lift an arms embargo to supply weapons to the Syrian opposition.
The commander of the rebel Free Syrian Army vowed to continue fighting until Assad's "criminal" regime is gone.
Salim Idris called on regime soldiers to join the rebels in a "fight for freedom and democracy". He said the opposition's task is "not easy" but pledged to continue despite his fighters suffering severe weapons shortages.
The country's banned Muslim Brotherhood group, holding a news conference in Istanbul, urged supporters to take part in a "week of action" to mark the two-year anniversary but did not specify what form this should take.
A Damascus-based activist who identified himself as Abu Qais said troops had increased patrols and security searches in the country's capital.
The uprising against Assad started with anti-government protests on March 15, 2011. The situation escalated as residents of the southern city of Deraa took to the streets after troops arrested teenagers who scrawled anti-regime graffiti on a wall.
The revolt has since morphed into a civil war with an estimated 70,000 people killed, according to the UN. A million Syrians have registered as refugees in neighbouring countries.
The International Committee of the Red Cross (ICRC) said on Friday that it was "deplorable" that people are getting used to the fact so many civilians are being killed each day in Syria.
The ICRC said that humanitarian aid efforts, though continuing, were falling short of what was required to take care of the millions of people affected.
"It is deplorable that high numbers of civilian casualties are now a daily occurrence to which people are unfortunately getting accustomed," said Robert Mardini, who heads the Red Cross' regional operations.
"These ongoing violations of international humanitarian law and of basic humanitarian principles by all sides must stop.
"Health standards have fallen dramatically, medical facilities have been targeted and health workers killed, intimidated or detained while trying to save lives," he said in a statement.
Mardini's remarks come a day after Amnesty International said the UN Security Council must refer war crimes committed by both sides in the conflict to the International Criminal Court.
"How many more civilians must die before the UN Security Council refers the situation to the prosecutor of the International Criminal Court so that there can be accountability for these horrendous crimes?" asked Ann Harrison,
Amnesty's deputy director for the Middle East and North Africa.
The rights watchdog has repeatedly accused both Assad's regime and rebel fighters of atrocities.
EU president Herman Van Rompuy said at the close of a two-day summit that leaders of the 27-nation bloc had discussed calls from Britain and France to abandon the embargo and "agreed to task our foreign ministers to assess the situation as a matter of priority" at the Dublin talks.