Backers of a Yemeni Islamist party that signed on to a plan to nudge the president from power scuffled with members of a Shiite rebel movement frozen out of the deal in the capital Sanaa on Tuesday, witnesses said.
They said loyalists of the Islah party traded blows with clubs and volleys of stones with members of the Houthi rebel movement, in the Sanaa square that has seen nearly a year of protests aimed at forcing the president from power.
Activists said at least 20 people were injured during the scuffles, which a representative of the Houthis said erupted when Islah backers attacked a tent they had set up to denounce the deal to edge President Ali Abdullah Saleh out of office.
That deal - crafted by Yemen's richer neighbours, and backed by a U.N. Security Council resolution and Washington - offers Saleh immunity from prosecution in exchange for handing power to his deputy, who is to work with a government including Islah and other opposition parties before a February presidential vote.
Saleh had backed out of the deal several times before signing it last month in Saudi Arabia, which shares U.S. fears that the struggle over Saleh's fate could descend into civil war and chaos that strengthens al Qaeda's wing in Yemen.
The deal did not include the Houthis, members of the Shiite Zaydi sect, whom Saleh launched an abortive campaign to crush - which also saw Saudi military intervention in 2009 - in the northwestern Saada province bordering Saudi Arabia.
U.N. Yemen envoy Jamal Benomar, who is attempting to shore up the transition deal, visited Saada this month and met with leaders of the Houthis, who have in recent months fought Sunni Islamists espousing Salafi doctrines that are influential in Saudi Arabia and condemn Shiites as heretics.
Washington is weighing a request for a visa from Saleh, who said he planned a U.S. visit on Saturday, hours after his forces killed nine people demanding he be tried for the deaths of hundreds of protesters during 11 months of protests against him.
He told reporters his trip would include medical tests. Saleh suffered burns and other injuries in an assassination attempt in June that capped fighting between his forces and those of a tribal faction influential in Islah.
The top "counter-terrorism" official in Washington - which wages a campaign of drone strikes against alleged al Qaeda members in Yemen and assassinated Anwar al-Awlaki, a U.S. citizen, that way earlier this year - urged Saleh's deputy Abd-Rabbu Mansour Hadi on Sunday to show restraint with protests.
Any successor to Saleh would face multiple, overlapping conflicts including renewed separatist sentiment in the south, which fought a civil war with Saleh's north in 1994 after four turbulent years of formal union.