Yemeni authorities stepped up security in the capital Sanaa on Friday ahead of rival rallies between government supporters and opponents, which the Interior Ministry said could be exploited by "terrorist elements".
Thousands of protesters calling for an end to President Ali Abdullah Saleh's 32-year rule have been holding a sit-in outside Sanaa University, while his supporters have rallied in another part of the capital.
Seventeen people have died in the past nine days in a sustained wave of nationwide anti-Saleh protests galvanised by the fall of the Tunisian and Egyptian presidents. Saleh has said he will not give in to "anarchy and killing".
An Interior Ministry statement late on Thursday ordered security forces to "raise their security vigilance and take all measures to control any terrorist elements" who might take advantage of the protests to infiltrate Sanaa.
Saleh, a U.S. ally against a Yemen-based al Qaeda wing that has launched attacks at home and abroad, is struggling to end month-old protests flaring across his impoverished country.
He is also trying to maintain a shaky truce with northern Shi'ite Muslim rebels and contain a secessionist insurgency in the south against northern rule.
Witnesses said police in Sanaa had formed cordons round the rival groups of protesters and supporters, whose numbers were expected to swell after Friday prayers, to prevent either side from confronting the other.
A statement from the Yemeni embassy in Washington said Saleh had "demanded security services offer full protection for the demonstrators" and prevent direct confrontations.
"Furthermore, the government calls on protesters to remain vigilant and take all precautionary steps to prevent the infiltrations of individuals seeking to carry out violent actions," the statement said.
State news agency Saba said Saleh has also assigned a committee headed by Prime Minister Ali Mohammed Megawar to open a dialogue with protesters to hear their demands