Security forces massed in the Yemeni capital on Friday ahead of rival demonstrations for and against the regime of embattled President Ali Abdullah Saleh, witnesses said.
Heavily armed Republican Guards and military vehicles were seen reinforcing troops already guarding the presidential palace, radio station and other key state buildings in Sanaa, an AFP correspondent said.
Tensions were high ahead of Friday's demonstrations arranged by both sides as they have done for the past three months on the day of weekly Muslim prayers.
Opposition activists dedicated the protests as a show of solidarity with the people of Saada in the north, where Zaidi Shiite rebels are based, while Saleh's supporters are due to mark Friday as a "Day of Unity."
Armed forces led by dissident General Ali Mohsen al-Ahmar have tightened a cordon around University Square, dubbed "Change Square," to protect camping protesters from possible attack by Saleh loyalists, witnesses said.
On Thursday night, Ahmar issued a statement condemning what he called the "brutal and barbaric" repression and called on government troops to disobey orders and refrain from firing at protesters.
Signup to our newsletter and follow us on Facebook and Twitter!
"Do not carry out orders from the regime to attack protesters and their peaceful demonstrations," he said, adding it was a "heinous crime not approved by any law."
The statement came after security forces and gunmen loyal to Saleh killed 19 people in a 24-hour period ending Thursday, prompting Washington to say the time has come for a transfer of power.
In Taez, Yemen's second largest city, protesters have announced more demonstrations.
The latest escalation came after neighbouring Gulf Cooperation Council states urged all sides in Yemen to sign a transition plan aimed at ending the political bloodshed.
Saleh has stalled by refusing to sign in his capacity as a president, insisting on endorsing the agreement as the leader of the ruling General People's Congress, contrary to the demands of the opposition.
But the United States, a long-time backer of Saleh, has thrown its support behind the GCC plan and urged Saleh to sign a deal "now."
Washington "is deeply concerned by recent violence throughout Yemen, and joins (the European Union) in strongly condemning these troubling actions," State Department deputy spokesman Mark Toner said on Thursday.
Around 175 people have been killed in the anti-government protests.