(CNN) -- Three people were killed and four others critically wounded Wednesday when Yemeni security forces shot live ammunition at protesters in the city of Taiz, according to doctors and eyewitnesses
"The attack took place while we were chanting, 'One more week left for you, Saleh,'" said Sabir Ali, a protester in Taiz. The protesters were giving President Ali Abdullah Saleh a week to step down, saying if he does not, they will make good on a vow to march to the presidential palace.
A senior security official denied that forces fired on protesters.
"Security forces were preventing attacks on civilians led by gangs of the opposition," the official said. "Security forces did not shoot at protesters. Gangs of the JMP were the shooters." The JMP stands for the joint meeting parties, a coalition of opposition parties.
"Security forces kill people, then deny the incident," said Ahmad Bahri, head of the political office at the opposition Haq party. "This is why the Yemeni revolution is taking place."
He said the protesters were unarmed and were marching peacefully when they were shot. Those who died were shot in the head, Bahri said.
Protesters calling themselves Revolution Youth had called for a civil disobedience day on Wednesday. More than 95 percent of the shops in Taiz were closed, according to eyewitnesses.
Civil disobedience was also seen in the Yemeni capital of Sanaa, along with the cities of Abyan, Aden, Ibb, Hodeida and Amran, according to reports.
On Monday, medical officials said at least six people were killed and hundreds wounded when teachers -- accompanied by opponents of Saleh's government -- demonstrated against salary cuts. Witnesses said security forces opened fire on the protesters and dispersed marchers with batons and tear gas before shooting at them.
CNN cannot independently verify the witness accounts.
Saleh's impoverished and unstable nation has been wracked by anti-government protests and clashes between demonstrators and security forces for many weeks.