Amnesty International has urged the Yemeni authorities to stop using excessive force to control anti-government demonstrations as continued violence against protesters across the country reportedly left several wounded.

At least six Yemeni protesters taking part in what appears to have been a peaceful sit-in were reportedly seriously injured in the city of Ta'izz today when security forces attacked them with what eyewitnesses described as a hand grenade, with dozens more also injured.

Meanwhile, activists in the capital Sana'a told Amnesty International today that they had been surrounded by security forces, aided by men described as "thugs", who were firing at them and issuing beatings.

"The Yemeni authorities seem to be stepping up their crackdown on protesters and we are gravely concerned that if that continues, the death toll will inevitably rise," said Philip Luther, Amnesty International's Middle East and North Africa Deputy Director.

"Yemen's government must allow people to peacefully assemble and protest."