Tanks and armoured vehicles have been deployed in Yemen's capital in the face of protests by tens of thousands of supporters of Shia rebels who are demanding the government change.
Protesters on Tuesday were responding to a call by Abdel Malek al-Houthi, the leader of the heavily armed Houthi group that has taken over a number of northern cities.
He has given the government until Friday to meet their demands of reinstating fuel subsidies and relinquishing power.
Yemen's supreme security committee, its most senior security body, gave warning that it would take "all measures to ensure the safety and security of the country,'' the AP news agency reported.
The committee listed what it said were "worrying signs'' of Houthi fighters on rooftops in some areas of the capital, Sanaa, as well as armed Houthi convoys entering the capital and setting up checkpoints.
Yemen's president, Abed Rabbo Mansour Hadi, said in televised remarks during a government emergency meeting that he would take "decisive and legal action'' against anyone disrupting the country's security, describing the demonstrations as "unacceptable.''
Military officials said that Yemen's elite "Presidential Forces'' were on standby in case of any attack, and were taking positions near government buildings, foreign missions and main intersections.
Ten of Yemen's international allies said in a joint statement that the Houthi actions were "antagonistic, militaristic and disrespectful".
"Threats of the kind you have made against the government are not a way to demonstrate any validity of your claims,'' said the statement posted on the website of the US embassy in Sanaa. It called upon Houthis to withdraw from the capital and hand over their weapons.
Mohammed Abdel-Salam, a Houthi spokesman, said the group rejected the "foreign mandate'' and vowed to continue "our blessed revolution peacefully".