Thousands of people have demonstrated in southern Yemen, demanding the release of a separatist leader.
Protest organisers said that some 8,000 people had marched in two cities after the authorities arrested Hassan Baoum. No reason was given for the detention.
Southern Yemen sees frequent unrest by separatists who claim that the central government discriminates against them.
The country is at the heart of a global security alert after two bombs were discovered on US-bound flights.
The bombs, sent via air freight to Chicago, were intercepted in Dubai and the UK on 29 October. Al-Qaeda in the Arabian Peninsula (AQAP) has said it was behind the parcel bomb plot.
Yemeni forces have stepped up security across the country in the wake of the global security scare.
Mr Baoum, 70, heads the Southern Movement, which organises frequent protests calling for independence or increased autonomy for the south.
He was arrested along with at least six people, including his son and three bodyguards, as they drove from Aden to the southern town of Daleh, local press reports said.
Abdallah al-Matari, a Southern Movement spokesman, told Al-Jazeera television that the arrest was an "immoral act that aimed to provoke the sons of the south".
Mr Matari said 5,000 people took to the streets on Wednesday in Daleh, while another 3,000 demonstrated in the nearby city of Radfan.
The protesters set tyres ablaze and blocked streets, reports said.
The former North and South Yemen formally united in 1990 but some in the south, where many of Yemen's oil facilities are located, complain that north Yemenis hold most of the key roles in local government, and that the bulk of the country's oil revenues go to Sanaa.
The government denies the charges.