At least nine people were killed after Syrian tanks mounted with machine-guns opened fire in the city of Homs, activists have said.
The shooting came in defiance of a day-old agreement between the Syrian government and the Arab League to end nearly eight months of bloodshed.
A crackdown on dissent and what appears to be growing sectarian bloodshed has turned Homs, Syria's third-largest city and home to some 800,000 people, into one of the country's deadliest areas.
The opposition said it will flood the streets on Friday to test whether the regime will stop using force against peaceful protesters.
"May Friday be the day where all streets and squares become platforms for demonstrations, and for the peaceful struggle toward achieving the downfall of the regime," said a Syrian activist coalition called the Local Coordination Committees.
The uprising shows no signs of stopping despite a government crackdown that the UN estimates has killed some 3,000 people.
The capture and death of Muammar Gaddafi last month only served to invigorate the Syrian protesters, many of whom carry signs and chant slogans warning President Bashar Assad that he will be the next dictator to go.
The latest bloodshed cast a pall over the Arab League accord announced in Cairo.
Under the plan, the Syrian government agreed to pull tanks and armoured vehicles out of cities, stop violence against protesters and release all political prisoners. Syria also agreed to allow journalists, rights groups and Arab League representatives to monitor the situation in the country.
A spokesman for the Syrian opposition in Cairo, Mo'men Kwafatiya, said Syria's approval of the Arab League proposal is a manoeuvre to avoid having its membership suspended or frozen in the Arab body, something that Gulf countries have quietly been pushing for.