A prominent member of the feminist group, Femen, in Tunisia has gone on trial in the city of Kairouan.
Amina Tyler is charged with carrying an "incendiary object". She appeared in court smiling, dressed in a white robe.
Conservative groups accuse her of insulting the city of Kairouan, a religious centre.
Ms Tyler scandalised some in her home country in March by posting photos of herself topless, with the slogan "my body is my own" written on her torso.
Femen, a Ukrainian group, is famous for its topless protests.
On Thursday, some 200 protesters, many religious conservatives, protested outside the court, chanting slogans against Ms Tyler, accusing her of attacking the city and insulting Islam.
Ms Tyler emerged from hiding earlier this month in Kairouan, where she wrote "Femen" on a wall near the city's main mosque.
That was on the same day as authorities banned a group of ultra-conservative Muslims from holding their annual conference in Kairouan.
Ms Tyler was detained by police amid clashes and tear gas as an angry crowd gathered.
Her lawyers argue that a 19th-Century charge of carrying an incendiary object should not apply to a can of pepper spray she says she had been given by a foreign journalist for her own protection.
Lawyers saying they represented the city called for her to face the more serious charge of threatening public security. They were turned down by the judge.
The trial has drawn attention partly because it is set against the background of tensions in Tunisia, following the overthrow of former President Zine El Abidine Ben Ali in 2011.
There has since been an increase in the prominence of ultra-conservative Islamists known as Salafists, who campaign for greater public piety in Tunisia.
Femen describes itself as "fighting patriarchy in its three manifestations - sexual exploitation of women, dictatorship and religion".
Ms Tyler's fellow activists around the world have staged topless protests calling for her freedom.