The opposition Syrian National Council (SNC) has elected a Kurdish activist as its new president.
Abdelbaset Sayda was chosen to replace Burhan Ghalioun, who has held the post since the group was established last September, at a meeting in Istanbul.
Activists have reported fresh shelling in Homs province, as international diplomatic efforts continue.
Britain's foreign secretary warned that Syria was beginning to resemble Bosnia in the 1990s.
William Hague said the situation was on the edge of a sectarian civil war, and said that he could not rule out international military intervention.
Meanwhile, details of attacks by the Free Syrian Army on government targets in Damascus on Friday have emerged, the BBC's Paul Danahar reports.
A power station came under attack, while a bus carrying Russian oil workers was also reportedly hit, our correspondent says.
Witnesses say many of the rebels involved were killed or captured. Casualties on the Syrian government side are not known, and the authorities in Damascus have not commented.
Speaking in Istanbul after the vote, Mr Sayda, who lives in exile in Sweden, told reporters he wanted to reform and restructure the Syrian National Council.
"We are entering a sensitive phase. The regime is on its last legs. The multiplying massacres and shellings show that it is struggling," he said.
Since its inception, the SNC has been plagued by divisions and complaints from activists that it is ineffectual.
Paris-based academic Mr Ghalioun stepped down last month after mounting criticism of his leadership.
There have also been accusations that the Council, which is an umbrella organisation for opposition groups, is dominated by Islamists and not inclusive enough.
Some hope that Mr Sayda, 56, could attract more Kurds and other Syrian minority groups to join forces with the council.
He has been given one month to convene another conference where the 300 or so members can sit down and agree to changes to the structure of the SNC.
The BBC's Jonathan Head in Istanbul says that as the Council thrashes out reforms at that meeting, they may well choose to elect another new leader.
After the meeting, outgoing president Burhan Ghalioun urged Russia to change its stance on the Syria situation.
"If the Russians are not yet convinced of the need for Mr Assad's departure, it means they have not yet found the way to realise the hopes of our people," he said.
Mr Ghalioun was reacting to comments made by Russia's Foreign Minister Sergei Lavrov on Saturday, who said Russia will continue to oppose attempts by the UN Security Council to sanction military intervention in Syria.
He said foreign powers were encouraging the armed opposition to hope that "the Libyan scenario" could be repeated.
On Sunday, activists said villages in Homs had been pounded by shells in a fresh assault on the restive province.
Abu al-Hoda, an activist in the village of Qusair, said women and children had been sheltering in basements of apartment buildings, too frightened to come out.
More than 80 civilians were killed by pro-government forces across the country on Saturday, activists said. The figures cannot be independently verified.
In London, Mr Hague said he did not know "how terrible" the situation would become, and that all options were still on the table.
"I think increasingly the analogy is not with Libya last year, but with Bosnia in the 1990s."
"We are on the edge of that kind of sectarian murder on a large scale, and so who knows what may be required from the international community to try to deal with that, if that gets going in that way."
He welcomed a Russian proposal for a international conference, but said he opposed the idea of Iran taking part.