The clash wounded one person and highlighted the tension in Yemen, where top generals, diplomats and tribal chiefs defected this week to the side of democracy protesters who have been camped out in central Sanaa for some six weeks.
Facing isolation, Saleh offered an amnesty to military personnel who have defected to the opposition and called for his resignation.
“I announce a general amnesty for those who committed foolishness before and after Monday. We consider it foolishness and a reaction to what happened on Friday," Saleh said in comments aired on Yemeni state television.
Saleh and opposition groups have both made proposals for reform. On Wednesday, Saleh offered new presidential elections by January 2012 instead of when his term ends in September 2013.
An umbrella group of civil society organizations called for a transitional council of nine figures "not involved with the corruption of the old regime" to draw up a new constitution over a six-month period ahead of elections. But the issue of what would happen to Saleh, who outlasted a civil war in 1994, a recent rebellion by northern rebels and separatist discontent in the south, was left untouched in the proposal from the group, called the Civil Bloc.
Opposition parties said Thursday they were tired of the drip-feed of concessions. "This talk is aimed at delaying the announcement of the death of the regime. The opposition does not need to respond," said spokesman Mohammed Al-Sabry.
The opposition is planning a "Day of Departure" for Friday after prayers that could bring hundreds of thousands onto streets. Around 10,000 people gathered on Thursday morning, chanting slogans against the president.
Authorities have withdrawn the licenses of Al Jazeera correspondents and ordered them to stop work, the Qatari channel said. Yemeni state media accuse the network of bias.
British Foreign Secretary William Hague said Britain has drawn up plans for a possible military evacuation of its citizens who remain in Yemen. Hague repeated a call for Britons in the country to leave while commercial flights were still operating and said he could not guarantee getting them all out safely were the crisis there to worsen.
"There are very detailed contingency plans, at very short notice to go into operation, for the evacuation of those British nationals that remain," he told Parliament.
"But that would have to be, if we have to trigger that, a military-only evacuation, possibly in very difficult circumstances so it would be difficult to be assured that we would be able to bring out everybody from remote parts of Yemen," he added.
Hague said he had reports that oil companies were withdrawing their staff.
In a fresh blow to Saleh, the leader of Yemen's largest tribe sided Thursday with opponents of the president, calling on Saleh to step down immediately and refrain from further violence against protesters.
The decision by the widely respected Sheikh Sinan Abu Lohoum, 80, was announced in a statement issued from the United States, where he is receiving medical treatment. It was read to protesters gathered at a central Sanaa square that has become the epicenter of the protests.
