SANAA, Yemen - Yemen's Shiite rebels announced on Friday that they will release a "constitutional declaration" - a move that will likely mark their formal takeover of the government and finalize a power grab that has been months in the making.

The rebels, known as Houthis, took control of the capital, Sanaa, in September, after descending from their northern stronghold and fighting their way into central Yemen, seizing several other cities and towns along the way.

Their escalating dominance - which included a raid of the presidential palace and a siege of President Abed Rabbo Mansour Hadi's residence - forced the president and all Cabinet members to submit their resignations in January.

Since then, Hadi and the ministers have been under house arrest. The rebels issued a deadline, which expired on Wednesday, for Yemen's political parties to negotiate what they called an "acceptable" way forward. Otherwise, they threatened they would act unilaterally.

The announcement about the constitutional declaration was broadcast on the rebels' television network, Al-Masseria TV. And text messages sent by the rebels told reporters the declaration would be released from the Republican Palace in Sanaa. The move would go against Yemeni law, which stipulates that only a president can issue constitutional declarations.

The Houthis, who are believed to be backed by Iran, called their supporters to take to the streets for evening celebrations, which are expected to follow the declaration. They also deployed armed men and pick-up trucks with anti-aircraft guns on main streets and around key institutions.

The development comes after days of failed talks sponsored by U.N. envoy to Yemen, Jamal Benomar.

According to senior politicians who attended the talks, the Houthis insist on the formation of a presidential council with representatives from northern and southern Yemen. Yemeni parties demanded assurances that the formation of the council will go hand-in-hand with a withdrawal of Houthi forces from key institutions and the release of Hadi and Cabinet members from house arrest.

Other parties in the talks wanted the parliament convene and possibly announce early elections, which the Houthis opposed, claiming the parliament had no legitimacy and that its mandate had expired.

Mohammed al-Sabri, a top politician from a multi-party alliance called the Joint Meeting Parties, described the Houthis' actions as a "coup," predicting it would lead to "international and regional isolation of Yemen."

Last year, the U.N. Security Council placed two Houthi leaders and deposed President Ali Abdullah Saleh - also believed to be a main backer of the Houthis - on a sanctions list for their role in derailing Yemen's transition.

"Today, the Houthis are taking an uncalculated," said al-Sabri. "They are a militia, not a political group."