Military checkpoints have been set up near Sanaa's airport
Yemen's security forces have been placed on a state of high alert amid fears of an al-Qaeda-linked attack.
In the capital Sanaa, tanks and troops have surrounded foreign missions, government offices and the airport.
Both the US, which closed 20 embassies worldwide on Sunday, and the UK have withdrawn diplomatic staff from Yemen and urged their citizens to leave.
Yemen has criticised the withdrawal, saying it served the interests of extremists.
The US embassy and consulate closures reportedly followed intercepted conversations between two senior al-Qaeda figures, including top leader Ayman al-Zawahiri, suggesting renewed terrorist attacks.
US special forces?
The BBC's Abdullah Ghorab in Sanaa says there are unprecedented security measures in the capital, with hundreds of armoured vehicles deployed.
He says a security source confirmed that Yemeni intelligence services had discovered that dozens of al-Qaeda members had arrived in Sanaa over the past days in preparation for a major attack.
The source suggested it was to include explosions and suicide attacks aimed at Western diplomatic missions and Yemeni military headquarters.
Checkpoints have been set up across the city and senior officials are being advised to limit their movements.
Amid the escalating tensions, sources also told BBC Newsnight that the US was preparing special operations forces for possible strike operations against al-Qaeda in Yemen.
Although the US has previously sent special forces to train counter-terrorist units, there are now suggestions that the Joint Special Operations Command (JSOC), may be preparing units for strike operations, the sources said.
JSOC co-operates closely with the CIA, which has mounted four drone strikes in Yemen over the past 10 days.
Yemen is the base of al-Qaeda in the Arabian Peninsula (AQAP) which has recently suffered a series of setbacks after the military launched an offensive in June with the help of US forces.
Both the White House and the US state department have said the current threat comes from AQAP but have refused to divulge further details.
According to the New York Times, the US intercepted communications between Zawahiri and the group's head in Yemen, Nasser al-Wuhayshi.
The paper said the conversation represented one of the most serious plots since the 9/11 attacks.
Yemeni officials quoted by AP news agency said they believed the motive for the latest threat was retaliation for the killing of senior AQAP figure Said al-Shihri, who was critically wounded in a November drone strike and later died of his wounds.
The Pentagon said the US Air Force began flying workers out of Sanaa on Tuesday morning.
In a statement on its website, the UK Foreign Office said: "Due to increased security concerns, all staff in the British embassy have been temporarily withdrawn and the embassy will remain closed until staff are able to return."
Several other European countries have also temporarily shut missions in Yemen.
The Yemeni government criticised the US and its allies for withdrawing embassy staff.
A foreign ministry statement read: "While the government of Yemen appreciates foreign governments' concern for the safety of their citizens, the evacuation of embassy staff serves the interests of the extremists."
It said it had taken all necessary measures to protect foreign missions.
Government spokesman Rajeh Badi said Yemeni forces had thwarted planned al-Qaeda attacks over the past 48 hours and that the international community "feared a reaction".
Mr Badi said the plans included attempts to take over two ports in the south and attacks on liquid gas stations in Shabua district.
Tuesday's evacuations came hours after a drone strike killed four suspected al-Qaeda militants in Yemen.
In a separate incident, militants shot down a Yemeni military helicopter over the al-Qaeda stronghold of Wadi Ubida in central Yemen, killing at least eight servicemen.