Stallholder Mohamed Bouazizi set himself alight on December 17, 2010 in a deadly protest over unemployment and police harassment that spiralled into the overthrow of dictator Zine El Abidine Ben Ali.
Tensions were running high as unemployed protesters and activists marched through the streets angry over the lack of jobs and opportunities that continue to plague Sidi Bouzid.
Some 40 members of radical Islamist group Hizb ut-Tahrir were detained for trying to hold an unsanctioned rally and unfurling banners critical of the authorities, security sources said.
On Saturday evening, security forces fired tear gas at demonstrators after they blocked some roads with tyres in the symbolic cradle of the revolution.
The mood was sombre at an official event to mark the occasion on Sunday, with allegations from organisers that the authorities were trying to downplay the key date in Tunisia's recent history.
The protests that began in Sidi Bouzid unleashed a wave of revolt that continues to reverberate around the Middle East.
Tunisia has emerged from the upheavals as the one relative success story and has been praised for its steps towards democracy, but it is still dogged by political and economic turbulence.