Yemen faced a dangerous power vacuum Friday after its president announced his resignation over a deadly standoff with Shiite militia controlling the capital and lawmakers called an emergency weekend session.
President Abdrabuh Mansur Hadi, a key US ally in the fight against Al-Qaeda, said late Thursday that he could no longer stay in office as the country was in "total deadlock".
Prime Minister Khalid Bahah also tendered his resignation, saying he did want to be part of the collapse of the country.
The fall of Hadi's Western-backed government would raise fears of complete chaos engulfing Yemen, strategically located next to oil-rich Saudi Arabia and on the key shipping route from the Suez Canal to the Gulf.
A senior State Department official said staffing at the US embassy in the capital Sanaa, already thin after most of the diplomatic personnel were ordered to leave in September, would be further reduced.
The country is an important power base for Al-Qaeda in the Arabian Peninsula (AQAP), which claimed responsibility for this month's deadly attack on French satirical weekly Charlie Hebdo.
Parliament is set to hold an extraordinary meeting to discuss Hadi's resignation offer, which needs to be approved by lawmakers to take effect.
Hadi advisor Sultan al-Atwani told AFP that parliament would meet on Sunday "at the earliest" because it is in recess and lawmakers need time to return.
Witnesses and security forces said that Huthi militiamen had encircled the parliament building overnight, having already seized the presidential palace earlier this week.
Gunmen have also surrounded the houses of top officials including Defence Minister Mahmud al-Subaihi and head of intelligence Ali al-Ahmedi, a security official said.
- Rival protests -
The Huthis, who hail from Yemen's northern highlands and who took control of most of the capital in September, said the constitution stipulated that Hadi's resignation should be approved by an absolute majority in parliament.
"As this has not happened, the resignation remains pending," the militia said in a statement.
Yemen has been riven by instability since an uprising forced strongman Ali Abdullah Saleh from power in 2012.
Saleh has been accused of backing the Huthis -- who are from the same Zaidi sect of Shiite Islam as the ex-leader -- as has Shiite-dominated Iran.
The Huthis, who have long complained of marginalisation, called on supporters to take to the streets on Friday afternoon to show their "backing for the revolutionary measures".
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Thousands of supporters answered the call, staging a rally north of Sanaa.
Meanwhile, hundreds others demonstrated near the University of Sanaa against the Huthis and in support of Hadi, witnesses said.
Similar protests were staged in the cities of Taez and Hudaida.
While the situation was generally calm in Sanaa, two small explosions targeted two houses belonging to Huthi members but there were no casualties.
After heavy fighting between government forces and the Huthis this week that killed at least 35 people, the UN Security Council and Yemen's Gulf neighbours had all voiced support for Hadi's continued rule.
The situation escalated on Saturday when the militiamen seized top presidential aide Ahmed Awad bin Mubarak in an apparent bid to extract changes to a draft constitution opposed by the Huthis because it would divide Yemen into six federal regions.
The Huthis continue to hold Mubarak and maintain a tight grip on the capital despite a deal struck late on Wednesday to end what authorities called a coup attempt.
In return for concessions over the disputed draft constitution, the Huthis pledged to vacate the presidential palace, free Mubarak, withdraw from areas surrounding the residences of Hadi and Bahah, and abandon checkpoints across the capital.
Hadi is from Yemen's formerly independent south and in recent days southern officials have taken steps to back his rule, including closing the air and sea ports in the main city of Aden.
The security and military committee for four of south Yemen's provinces, including Aden, said late Thursday it would not take orders from Sanaa following Hadi's resignation.
UN Secretary General Ban Ki-moon called on all sides "to exercise maximum restraint and maintain peace and stability", his spokesman said.
His envoy to Yemen, Moroccan diplomat Jamal Benomar, had arrived in the country on Thursday for talks with the political rivals, but they were swiftly overtaken by events.
In Washington, State Department spokeswoman Jen Psaki said the United States was assessing the fast-moving events.
"We continue to support a peaceful transition. We've urged all parties and continue to urge all parties to abide by... the peace and national partnership agreement," Psaki told reporters.
Yemen has allowed the United States to carry out repeated drone attacks on Al-Qaeda militants in its territory.
Oxfam warned that 16 million people -- more than half the population -- were in need of aid in Yemen.
"A humanitarian crisis of extreme proportions is at risk of unfolding in the country if instability continues," the aid group said.