President Abd Rabbu Mansour Hadi has urged Yemen's armed forces to "raise their level of vigilance" on the eve of the expiry of a rebel deadline for the government to quit.
Bracing for escalation during the weekly Friday prayers, Hadi on Thursday urged the armed forces to "be prepared to deal with any development".
He made the call at a meeting of the Supreme Security Committee and the National Defence Council, the official Saba news agency said.
Rebels roamed Sanaa, the capital, on Thursday in vehicles urging residents via loudspeakers to join their demonstrations, witnesses said.
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Also on Thursday, Hadi sent a delegation to Shia Zaidi rebel commander Abdulmalik al-Huthi in his Saada stronghold north of Sanaa to invite his group to talk and urge them to join a national unity government.
The rebel commander said the presidential delegation had "shown its understanding of some of our demands", but that negotiations and protests will continue.
"We will continue our pressure and legitimate movement as we have not yet reached a clear and sufficient response to our demands," he said, urging Yemenis to gather for Friday prayers on the road to Sanaa Airport.
He insisted that their movement will employ "peaceful, civilised and legitimate means".
He urged Hadi and Defence Minister Mohammed Nasser Ahmed to "not fall into the trap of attacking people and protesters".
Huthi had warned on Sunday that the authorities must address protesters' grievances by the end of the week or face "legitimate action".
Tensions high in capital
Thousands of armed Shia rebels, also known as Ansarullah or Huthis, strengthened their positions in Sanaa on Wednesday as they pressed their campaign to force the government, which they accuse of corruption, to step down.
The protesters also want a steep increase in petrol prices to be revoked.
The capital was gripped by fear and "residents are storing food supplies as others get weapons", a young student in Sanaa told the AFP news agency.
The Zaidi rebels strongly oppose the government's plans for a six-region federation, demanding a single region for the northern highlands and a greater share of power in the federal government.
They control Saada province in the far north and parts of several neighbouring provinces.
Rebels reached the outskirts of Sanaa in July after seizing Amran to its north, although they later agreed to withdraw.
The rebels have been fighting an on-off conflict with government troops in the northern mountains for the past decade, but analysts warned their bid for more power in a promised new federal Yemen was potentially explosive.
The Zaidi Shias are a minority in mainly Sunni Yemen but the majority in the northern highlands, including the Sanaa region.