Hundreds of armed Shiite rebels staged sit-ins on Tuesday on the outskirts of capital Sanaa, where supporters are mobilising in a campaign for the fall of the government by Friday.
Activists in the Ansarullah, or Huthi, rebellion put up dozens of tents at the western edge of Sanaa.
Guarded by hordes of armed men, other rebels were busy erecting similar camps in the north and south of the city, AFP journalists reported.
Around 5,000 men arrived in the capital from Saada province, traditional stronghold of the Ansarullah rebellion.
Rebel leader AbdulMalik Huthi on Sunday ordered his followers to march on Sanaa to bring about "the fall of the government, which has failed."
He set the authorities a deadline of Friday before other forms of "lawful" action will start. He gave no details.
Huthi's supporters are opposing a recent sharp increase in petrol prices, which have a major impact on household budgets in the impoverished country.
Tens of thousands of Shiite rebels demonstrated in Sanaa city centre on Monday.
The programme of protests will go on until Friday and "we will not yield", a rebel told AFP.
"President Abdrabuh Mansur Hadi should listen to the voice of the people, or we will overthrow this corrupt goverment which has starved the people," the militant said as he took part in one of the rebel sit-ins.
Security forces have stepped up their presence at the western edge of Sanaa.
"We have been charged with ensuring safety and preventing armed groups from entering Sanaa," Abdulghani Tajeddin, commander of a local unit, told AFP.
"We will confront any armed group seeking to harm the country's security and stability."
The face-off could degenerate if the rebels seek to escalate their action, political sources said.
"We won't accept the status quo that the Huthis want to impose on us by force of arms," warned Mohamed al-Sabri, head of a political coalition including Islamist party Al-Islah, a Sunni bloc.
"If they make an attempt to enter Sanaa... it will be suicide for them," he said.
Ansarullah controls Saada province in northern Yemen and is suspected of wanting to broaden its sphere of influence in a future federal state, potentially comprising six provinces.
Huthi forces reached just outside Sanaa in July when they took the city of Amran, although they later agreed to withdraw.