Syria braced for new demonstrations on Friday after weekly Muslim prayers as US President Barack Obama piled pressure on Damascus to curb a brutal crackdown on pro-democracy protests.
The opposition called for massive demonstrations across the country after the mid-day prayers, promising a "surprise" for President Bashar al-Assad's regime in the two major power centres, Damascus and Aleppo.
"People are ready for this Friday," said the Syrian Revolution 2011, a Facebook group spurring anti-regime protests.
"Damascus and Aleppo are preparing a surprise for the regime and the shabiha," it added in reference to pro-government "thugs," without elaborating.
"We will not be tolerant with the security forces or shabiha," it said. "We won't let them arrest us and we will be like a thorn in their throat."
Crucially, both Damascus and Aleppo have so far been largely spared the unrest roiling Syria and it is widely believed that should massive demonstrations begin there that would mark a serious setback for the authoritarian regime.
Friday's protests were called amid mounting pressure by the international community for the Syrian government to stop its crackdown on demonstrators who have taken to the streets, emboldened by revolutions in Tunisia and Egypt.
In a keynote speech Thursday on the Middle East, Obama urged his Syrian counterpart Assad, who is facing the greatest challenge to his 11-year rule, to lead a political transition or "get out".
"President Assad now has a choice," Obama said in his speech. "He can lead that transition or get out of the way.
"The Syrian government must stop shooting demonstrators and allow peaceful protests."
Damascus, however, defiantly rejected the warning, countering that Obama's appeal was not aimed at easing tensions in Syria but rather at sowing discord.
"Obama is inciting violence when he says that Assad and his regime will face challenges from the inside and will be isolated on the outside if he fails to adopt democratic reforms," the official news agency SANA said.
The government newspaper Al-Thawra also criticised the US president saying: "He (Obama) didn't forget his arrogance in telling a sovereign country what to do ... and threatening to isolate this country if it fails to do as told."
More than 850 people have been killed and thousands arrested since the protests began in mid-March, according to human rights groups and the United Nations.
Assad's government has blamed the violence on "armed terrorist gangs" backed by Islamists and foreign agitators.
A confident Assad earlier this week said he believes the unrest was coming to an end and, in an unusual step, acknowledged wrongdoing by the country's security services.
The protests have posed the greatest challenge to nearly five decades of rule by his Baath party, which is controlled by members of the minority Alawite community, an offshoot of Shiite Islam.
The majority of Syria's 23-million population are Sunni Muslims.
Western powers initially were hesitant to criticise Assad's regime due to Syria's strategic importance in the region and fears of possible civil war if the regime were to collapse.