Authorities in Yemen have on Sunday arrested the leader of student rallies against the government in the capital Sanaa.
Tawakul Karman led two protests at Sanaa University, criticising Arab leaders and calling on Yemenis to topple Ali Abdullah Saleh, the president, by text messaging and email.
A security source told Reuters news agency on Sunday that Karman, a member of the Islah party, was arrested by orders from the General Prosecution Office.
Mohamed Ismail al-Nehmi, her husband who was with her, also told Reuters that police stopped Karman on her way home early on Sunday and charged her with organising unlicensed demonstrations without permission.
"I have no accurate information about her whereabouts," he said.
"Maybe at the central prison, maybe somewhere else, I don't know."
After Karman's arrest, several hundred students gathered outside Sanaa University, demanding her release.
Some 50-60 policemen armed with shields and batons stopped the crowd, which was chanting "release the prisoners", from marching towards the general prosecutors' office, the witness said.
Karman, who heads the Yemeni activist group Women Journalists Without Chains, had also called on Yemenis to support the Tunisian people.
Her arrest comes a day after thousands of Yemeni students, activists and opposition groups held protests at Sanaa University, demanding Saleh's ouster in what appeared to be the first large-scale challenge to the strongman.
Around 2,500 students, activists and opposition groups chanted slogans against the president, comparing him to Tunisia's ousted President Zine El Abidine Ben Ali, whose people were similarly enraged by economic woes and government corruption.
"Oh, Ali, join your friend Ben Ali," protesters chanted.
Police fired tear gas at the demonstrators, whose grievances include proposed constitutional changes that would allow the president to rule for a lifetime.
Since the Tunisian turmoil, Saleh has ordered income taxes slashed in half and has instructed his government to control prices. He also ordered a heavy deployment of anti-riot police and soldiers to several key areas in the capital and its surroundings to prevent any riots.
Nearly half of Yemen's population lives below the poverty line of $2 a day and does not have access to proper sanitation. Less than a tenth of the roads are paved. Tens of thousands have been displaced from their homes by conflict, flooding the main cities.
The government is riddled with corruption and has little control outside the capital. Its main source of income - oil - could run dry in a decade.
Protests were also held in the southern port city of Aden, where calls for Saleh to step down were heard along with the more familiar slogans for southern secession. Police fired on demonstrators, injuring four, and detaining 22 others in heavy clashes.
While some students protested against Saleh, others affiliated with his General People's Congress demonstrated in his support, with banners calling for him to remain in office, and for parliamentary elections to be held in April.
Saleh said in December that parliamentary polls would take place in April with or without opposition parties, some of which have said they are considering boycotting the election.