UN sanctions against Ali Abdullah Saleh, Yemen's former president, and two Houthi rebel leaders are due to take effect within hours, as pro-Saleh protesters angry at the move took to the streets of Sanaa, the capital.
The US-proposed sanctions against Saleh, Abd al-Khaliq al-Houthi and Abdullah Yahya al-Hakim, are due to come into effect at 22:00 GMT on Friday, provided no member of the UN Security Council objects to them, UN sources told Al Jazeera.
The three men are blamed for orchestrating Yemen's current unrest.
The sanctions include a travel ban and assets freeze on the three men, diplomats in New York said earlier in the week.
In Sanaa, thousands of supporters of Saleh, along with Houthi fighters who have overrun the capital, have rallied, denouncing what they say is US "interference'' in Yemen's affairs.
The protesters carried posters urging the US ambassador to get out of the country, condemning the proposed sanctions against the three men.
"The protesters believe [President Abd Rabbuh Mansur] Hadi is not the legitimate president of the country. They say he is the US's and UN's president," Peter Salisbury, a journalist and analyst, told Al Jazeera from Sanaa.
"There is a major power vacuum in Yemen. For the last three weeks, the prime minister has been trying to form a government, but still, there is no sign of success. What we are likely to see tomorrow [Saturday] is more protests."
Saleh served as Yemen's first president after unification in 1990 before being forced to step down in February 2012 under a regional peace plan.
However, the former president is seen as prime backer of the rebel Houthi movement that seized the capital Sanaa in September and has since spread its control into central and west Yemen, in defiance of a UN peace plan.
Abd al-Khaliq al-Houthi is the younger brother of chief rebel leader Abdulmalik al-Houthi and was among commanders who oversaw the storming of Sanaa. Hakim is Abdulmalik al-Houthi's military second-in-command.
Saleh's office said on Wednesday that he has been given a deadline of 14:00 GMT on Friday to leave Yemen or face possible sanctions. Washington rejected the claim on the same day.
The UN Security Council in August called on the Houthi rebels to end their armed uprising against President Hadi and had warned of sanctions against those who threaten the stability of Yemen, which is a key US ally in the fight against al-Qaeda.
The turmoil has raised fears that the impoverished country, which neighbours major oil-producer Saudi Arabia and lies on the key shipping route from the Suez Canal to the Gulf, may become a failed state.
The Shia Houthis have been able to expand their territory largely unchallenged by government forces, taking control of the capital, the key port of Hudeida, and the provinces of Ibb and Dhamar.
The only real resistance they have faced has come from Sunni tribes and al-Qaeda fighters.
Source: Al Jazeera and agencies