Syrian opposition activists have called for mass rallies in support of the Free Syrian Army, a group of army defectors seeking to topple the government.
Demonstrations are reportedly already taking place in Aleppo, Deir al-Zour, Homs, Idlib and suburbs of Damascus.
One activist group said two civilians had been killed, one of them a child.
On Thursday, the Free Syrian Army and the main opposition coalition, the Syrian National Council, agreed to co-ordinate their operations more closely.
An SNC statement said a liaison office would be set up with the FSA to "maintain direct communications around the clock".
The groups also agreed to devise a plan, which would include "the reorganisation of FSA units and brigades, and the creation of a format to accommodate within FSA ranks additional officers and soldiers, especially senior military officials, who side with the revolution", the SNC added.
The SNC initially opposed the use of force in the uprising against President Bashar al-Assad, while the FSA operated independently.
It is impossible to verify how many army defectors have joined the FSA, but its leader, Col Riyad al-Asaad, has put the figure at as many as 20,000.
The group has said it is behind an increasing number of attacks on Syrian security forces, and the authorities have acknowledged mounting losses.
The government says 2,000 security personnel have died combating "armed gangs and terrorists". The UN last month said more than 5,000 people had been killed by Syrian security forces since the uprising began in March.
Also on Thursday, the secretary general of the Arab League defended the organisation's observer mission in Syria, saying in an interview with the BBC that it had helped to save lives.
Nabil al-Arabi was responding to criticism of the mission, which one former monitor has called a "farce".
Mr Arabi said the presence of the observers had encouraged more Syrians to take part in peaceful demonstrations.
"The observers are in Damascus to verify that shooting and killing has stopped. This has not materialised. So, the rationale for sending observers has not materialised," he said.
He added that he regretted President Assad's criticism of the Arab League in a speech this week and hinted they had exchanged sharp words in private.
Meanwhile, Syrian border guards turned back several hundred activists who wanted to take humanitarian aid across the border from Turkey. The guards said they did not have the right permits to enter the country.
The activists, who called themselves the Freedom Convoy, said they would stage a sit-in protest close to the border.