|Protesters responded to the attack by hurling rocks at the security forces [EPA]
Security forces and armed men in civilian clothes have opened fire during protests in the city of Taiz in southern Yemen, a day after clashes there killed 15 people, witnesses said.
Hundreds of security troops attacked tens of thousands of protesters, witnesses told the Reuters news agency on Tuesday, adding that plain-clothed policemen were wielding bats and daggers.
They said several people had been hurt, but there was no word yet from medical sources on casualties.
Protesters responded to the attack by hurling rocks at the security forces.
Our special correspondent, reporting from Yemen, quoted the opposition that the ruling party had called on their supporters to rally in Taiz. The supporters marched towards "Liberty Square".
"They tried to enter the square but anti-government protesters stopped them after which clashes ensued. The police then opened fire and lobbed tear gas shells towards the protesters inside the square.
"As events are unfolding, it is strengthening their (the protesters) resolve."
Violence in Sanaa
In Sanaa, pro-regime supporters marched from the presidential area towards Change Square and the headquarters of General Ali Mohsen, a key military leader who recently threw his weight behind the tens of thousands of protesters calling for Ali Abdullah Saleh, Yemen's president, to quit.
"They were on foot and in cars. As they approached the gate of the (Mohsen) HQ, people in one of the cars opened fire on the soldiers as well as the pro- and anti-government supporters who were gathered there," added our correspondent.
"Ali Mohsen's soldiers, loyal to the pro-democracy protesters, responded with fire. We are hearing that one person got killed, 26 others injured of which four are in a critical condition.
"It is a clear indication of escalation; the president is not backing down and the situation remains very volatile - he seems to be increasing the security presence, not decreasing it."
The official news agency SANA said four people had been killed and 56 injured.
Meanwhile, Yemen has accepted an invitation by Gulf Arab states to hold talks on its weeks-old political crisis in Saudi Arabia as a drive to oust Saleh from his three-decade rule gathered pace.
The Gulf Cooperation Council (GCC) invited the Yemeni government and opposition representatives for talks in Riyadh in a bid to end the crisis, but a date has yet to be set.
"We welcome the GCC invitation and the government is ready to discuss any ideas from our Gulf brothers to solve the crisis," Abubakr al-Qirbi, Yemen's acting foreign minister, told Reuters.
Aides to Mohsen said the general also accepted the call for talks in Saudi Arabia.
But leaders from the political opposition groups have yet to respond, saying they would only give an answer when they receive details of the proposed talks in Riyadh.