Unidentified gunmen firing on an anti-government rally in the Yemeni capital Sanaa have killed at least 39 people and injured 200, doctors told the BBC.The gunmen, said to be wearing civilian clothes, fired from roof-tops overlooking the central square.
State media say the security forces had nothing to do with the violence and blamed it on "clashes among citizens".
President Ali Abdullah Saleh has declared a state of emergency across the country.
Speaking after a month of violence which rocked Yemen, he reportedly expressed sorrow for the violence in Sanaa, which began after Friday prayers.
Yemen's opposition condemned the shooting and urged Mr Saleh to leave.
"There is no longer any possibility of mutual understanding with this regime and he has no choice but to surrender authority to the people," Yassin Noman, rotating president of Yemen's umbrella opposition group, was quoted by Reuters news agency as saying.
According to Reuters, tens of thousands of protesters also gathered in other cities across the country, from the southern port of Aden to Hodeida in the west.
"Most of the wounds were to the head, neck and chest," one doctor told AFP about Friday's shooting.
Yemen is one of a number of countries in the region that have seen unrest since the presidents of Egypt and Tunisia were ousted in popular revolts.
Middle East unrest: Yemen
- President Ali Abdullah Saleh in power since 1978
- Population 24.3m; land area 536,869 sq km
- The population has a median age of 17.9, and a literacy rate of 61%
- Youth unemployment is 15%
- Gross national income per head is $1,060 (£655) (World Bank 2009)
Thousands of people have turned out for regular demonstrations in cities including Sanaa, Aden, Taiz and elsewhere, calling for corruption and unemployment to be tackled and demanding the president step down.Some 40% of the population live on $2 (£1.20) a day or less in the country, and a third face food shortages.
The protests have often been met by riot police or supporters of President Saleh armed with knives and batons.
On 9 March one person was killed and at least 80 were injured when forces opened fire on a similar protest by a group which has been camped out in front of the university since mid-February.
The president has been in power for 32 years, facing a separatist movement in the south, a branch of al-Qaeda, and a periodic conflict with Shia tribes in the north.
He has said he will not seek another term in office in 2013 but has vowed to defend his regime "with every drop of blood".