Under the slogan "Hama, forgive us," the activists appealed for "millions" of people to take part in the demonstrations in honour of those who died in the massacre ordered by the father of President Bashar al-Assad.
They were urged to dress in black for the demonstration after the main weekly Muslim prayers by The Syrian Revolution 2011, an Internet-based activist group that has been the main motor of a popular uprising that erupted almost 11 months ago.
The regime of Hafez al-Assad, the late father of the current president, crushed an Islamist revolt in Hama in 1982, killing an estimated 10,000 to 40,000 civilians, according to international rights groups.
Rallies were staged in memory on Thursday as Western and Arab members of the UN Security Council dropped an explicit call for President Assad to quit in a bid to win Russian backing.
Activists in Hama itself defiantly painted roads red to symbolise blood and staged a general strike, the Syrian Observatory for Human Rights said.
The city's famed waterwheels carried a message that read "Hafez is dead while Hama has not disappeared," the Britain-based Observatory said.
Hama has again been targeted by security forces in a nationwide crackdown on dissent that the Observatory estimates has killed more than 6,000 people since the uprising broke out in March.