Troops loyal to Yemeni President Ali Abdullah Saleh deployed in force in Sanaa on Sunday after opposition groups called new mass demonstrations demanding his ouster, an AFP correspondent reported.
The security forces closed off all access to the capital from Saturday afternoon while armed civilians loyal to the president also took to the streets, the correspondent said.
Sanaa has been without electricity since Saturday afternoon, while most fuel stations in the capital shut their taps suddenly, causing chaos at the few outlets which were still serving motorists.
The massive deployment by Saleh loyalists came in response to an opposition call for an intensification of protests against his rule amid a deadlock in the political process in the face of the president's long absence abroad.
Saleh has been receiving medical treatment in neighbouring Saudi Arabia for wounds sustained in a June 3 bombing in his palace.
"We have called for intensifying the challenge in order to move towards a peaceful solution," said Huria Machhur, spokeswoman of the opposition National Council, an umbrella group of anti-Saleh forces.
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"The political process has reached an impasse because of Saleh's refusal to sign the Gulf plan," she said, adding that popular protests which began in January should continue "until the fall of the regime."
The Gulf plan proposes that Saleh transfer power to the vice president within 30 days in exchange for a promise of immunity from prosecution.
Saleh vowed last month to return "soon" to his impoverished country.
"We hope that forces loyal to Saleh do not use weapons to disperse the peaceful marches of young people," Machhur said, warning that army units which defected to the protest movement "are on alert to defend the protesters if they face acts of violence."
"We hope that there is no challenge... to avoid an armed confrontation with dangerous consequences," she said, urging the Gulf countries, the United States and the European Union "to increase pressure on the regime" to avoid a civil war.
Meanwhile, armed tribesmen clashed with Saleh loyalists in the elite Republican Guards at dawn on Sunday both in the south and in Taiz, Yemen's second-largest city, residents said.
Yemen's ruling party, the General People's Congress, accused the Common Forum parliamentary opposition bloc -- the main component of the National Council -- of a "plot" to "take power by force" by mobilising young protesters.
In a statement, it said it holds the opposition "responsible for the consequences" of an escalation of violence and called for the crisis to be resolved through a "serious and responsible dialogue."