ADEN: Police arrested two separatist leaders in south Yemen ahead of planned rival rallies marking the first anniversary of ex-president Ali Abdullah Saleh's ouster, an official and activists said Wednesday.

Qassem Askar, a leader of the hardline faction of the separatist Southern Movement, was arrested early in the morning in southern port city Aden, activist Yasser al-Yafie told AFP. "He was taken to an unknown location."

Southern cleric Hussein bin Shouaib was arrested late Tuesday after he chaired a meeting urging protests in Aden, the ex-capital of the formerly independent south, a security official said.

The arrests come on the eve of the first anniversary Thursday of the uncontested election of President Abdrabuh Mansur Hadi which ended Saleh's 33-year autocratic rule, following a year-long deadly uprising to topple the former president.

Supporters and opponents of Hadi, himself a southerner, have said they will hold rallies on Thursday, prompting fears of possible clashes in Aden -- a stronghold of southern separatists.

The Islamist Al-Islah (reform) Party has called for a pro-Hadi rally in Aden's Khor Maksar district.

"We believe that the wise ones among the Southern Movement will not use violence against a peaceful and civilised gathering," said Abdullah al-Alimi, a youth activist linked to Al-Islah.

The powerful separatist Southern Movement has warned of unrest if Al-Islah goes ahead with demonstrations while the head of the group, Hassan Baoum, called on supporters to hold anti-Hadi rallies in Aden.

Baoum called for massive rallies in Aden "to reject the election" that brought Hadi to power and pursue the "struggle until liberation" of the south.

"We have already warned Al-Islah from holding any gatherings in Aden," said Naser al-Khabji, a leader of the Southern Movement who also warned "clashes could break out" if pro-Hadi supporters demonstrate in Aden.

Amnesty International called on security forces to avoid violent repression of protests in the south.

"The Southern Movement and its followers have a right to protest peacefully, and the Yemeni authorities must allow them this right," Ann Harrison, Amnesty's deputy director for the Middle East and North Africa, said in a statement.

"That means that security forces deployed to police these demonstrations must refrain from using excessive, lethal force against peaceful protesters, something they have failed to do in the recent past."

South Yemen was a separate state before unification with the north in 1990. It broke away four years later, sparking a civil war, before it was overrun by northern troops.

Some factions of the Southern Movement want autonomy for the area, but more hardline members are pressing for a return to complete independence for the south where residents complain of discrimination by the Sanaa government.