The Huthis belong to the Zaidi sect, an offshoot of Shiite Islam which makes up about a third of the population of the Sunni-majority country.
Zaidis are the majority in Yemen's northern provinces bordering Sunni powerhouse Saudi Arabia, and the Huthis face allegations of support from Shiite-dominated Iran.
During protests, Huthi supporters chant Iran's famous Islamic revolutionary slogan "Death to America! Death to Israel!"
Ali Abdullah Saleh, the former strongman who stepped down in 2012 after an uprising, has also been accused of backing the Huthis.
The Huthis oppose a plan for a six-region federation, saying it would divide the country into rich and poor areas.
They have long complained of marginalisation and fought six wars with the government between 2004 and February 2010, when they signed a truce.
Thousands of people were killed in the rebellion.
The north was a Zaidi imamate until a 1962 coup turned Yemen into a republic ruled by a government considered illegitimate by the Huthis.
The militiamen, who in 2013 renamed themselves Ansarullah (Supporters of God), claim direct descent from the family of the Prophet Mohammed.
They are named after their late spiritual leader Badreddin al-Huthi, who formed the "Faithful Youth" movement in 1992 as a political group to fight discrimination.
His son Hussein led a nearly three-month uprising in the northwestern province of Saada before the army killed him in September 2004.
The militia is now led by Hussein's brother Abdulmalik al-Huthi.
The Huthi chief said in a televised address Tuesday that "all options" were open against President Abdrabuh Mansur Hadi, whom he accused of supporting the "fragmentation" of the country.
Role in 2011 revolt
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As an Arab Spring-inspired uprising against Saleh swept through Yemen in 2011, the Huthis reached out to the opposition in the capital Sanaa and joined protest camps there.
It was their first major show of influence outside their northern bastion in Saada and Amran provinces.
However, the Huthis rejected a deal brokered by Saudi Arabia, which fought them between 2009 and 2010 after a border incursion.
Under the deal, Saleh, himself a Zaidi who ruled Yemen for 33 years, was replaced as president by Hadi in 2012 and a consensus government was formed.
The Huthis rejected the new government and repeatedly accused it of corruption.
Advance on Sanaa
In early 2014, the Huthis drove the Al-Ahmar tribe out of its bastions in Amran, north of the capital, seeking greater clout within a planned federal Yemen.
On August 18, they established camps inside Sanaa and clashed with government allies as Hadi branded their offensive a "coup attempt".
On September 21, the Huthis took over key institutions, including the government headquarters and military sites, after an apparent surrender by security forces.
They signed a UN-backed agreement on the same day under which a new government was formed. It stipulated the withdrawal of the Huthi forces from Sanaa -- a pledge they have yet to fulfil.
Since seizing the capital, the Huthis have pressed their advance south of Sanaa, where they have met stiff resistance from Sunnis, including Al-Qaeda loyalists.
Since Saturday, the Huthis have stepped up their challenge to Hadi, kidnapping his chief of staff Ahmed Awad bin Mubarak.
Late on Monday, they encircled the residence of Prime Minister Khalid Bahah and the next day seized the presidential palace, in a push to extract changes to a draft constitution including the six-region plan.
Their actions were condemned by the UN Security Council, which had already sanctioned two Huthi military commanders as well as Saleh.