SANAA (Reuters) - When riots erupted this month over an anti-Islam film made in California, Houthi rebels, long confined to remote corners of Yemen by then-President Ali Abdullah Saleh, covered the capital Sanaa in posters, banners and graffiti denouncing the United States.
Western diplomats and Sunni Islamists were taken aback by the sudden show of strength in a city from which the Shi'ite rebel group had long been banished.
"They really have a very broad strategy to increase their influence on politics and society - we believe for very negative purposes," said one senior Western diplomat.
"My take on the rhetoric is that it is bought and paid for by the government of Iran and they are simply following instructions that they get from Tehran."
Analysts and diplomats believe that the ascent of the Houthis, named after its leaders' family, has turned Yemen into a new front in a long struggle between Iran and Western powers and the Arab regimes they back, centred on a nuclear programme that Israel and the West say is aimed at making atomic weapons and altering the regional balance of power. Iran denies those charges.
Gulf Arab governments and Sunni clerical allies accuse Iran of backing Shi'ite communities around the region, and Sanaa this year also accused Iran of trying to meddle in Yemeni affairs.
Saleh's successor Abd-Rabbu Mansour Hadi snubbed a visiting Iranian envoy in July to signal "displeasure" with Tehran after Sanaa said it had uncovered an Iranian-led spy ring in the capital, charges Hadi repeated during a visit to the United States last week.
In Sanaa, whose large Zaydi Shi'ite population, especially in historic Old Sanaa, forms a natural support base for the Houthis, slogans attacking the United States borrow directly from Iranian lexicon such as "the great Satan" and "global arrogance".
Others call for a boycott of U.S. and Israeli products, which the group believes find their way disguised into Yemen.
Other slogans declare that the graffiti campaign is a kind of revolutionary tactic, declaring that "slogans have revealed the high level of alert, caution and readiness to the nation".
The slogans started to appear after protesters stormed the U.S. embassy last month, followed by street protests that security forces allowed to peter out without incident.
"The government wants to show more freedoms and doesn't want any clash, so it left them alone," says Abdulfattah, an Arabic teacher in the old city who opposes the group. "They found a chance to show themselves and exploit people's feelings."
The interim government aims to include the Houthis in a national dialogue set for November which is meant to work out a new pluralistic political system.
But some Sunnis fear the Houthis want to revive the Zaydi Imamate, the 1,000-year-long rule of Yemen in which power was passed through leaders claiming descent to the Prophet Mohammed. The imamate ended in a 1962 military coup.
In the complex interweaving of tribal and religious boundaries in Yemeni society, sympathies do not split along purely Sunni/Shi'ite lines, however.
Some Sunnis who also claim descent from the Prophet Mohammad are sympathetic to the Houthis, while many Zaydis, including Saleh and his Al-Ahmar clan, opposed them.
The Al-Ahmar family itself dominates the Hashid tribal federation, which comprises both Sunnis and Shi'ites, and the Al-Ahmars are key figures in the Sunni Islamist Islah party.
Many Zaydis in the old city have acquiesced to the growing power of the Houthis, however.
"This is people expressing themselves; 30 or 40 percent of the Old City are sympathetic to it," said Ahmed Ezz, outside an old city mosque, adding: "Houthi thought isn't extremist."
Analysts say the Houthi phenomenon - like the emergence of southern secessionism - is a result of marginalisation and the hold that Saleh's northern-based and Saudi-backed tribal and religious ruling elite had on power and economic resources.
The group emerged after a civil war in 1994 enforced unity between north and south Yemen, evolving into a Hezbollah-style militia centred around the Houthi family in the remote and neglected Saada province bordering Saudi Arabia.
Saleh's failed wars to crush them began in 2004.
Calling themselves Ansarallah (Partisans of God), a term apparently modeled on Hezbollah (Party of God) in Lebanon, the Houthis have built alliances with the southern secessionist movement and run a TV channel al-Maseera out of Beirut.
They taunt their Sunni rivals as pawns of Riyadh and puritanical Wahhabi Islam who are not willing to challenge U.S. policy in the region.
"Wahhabi thought came from Saudi Arabia about 50 years ago, but Houthi thought has been in Yemen for over 1,000 years," said Ali al-Emad, leader of Ansarallah's youth section in Sanaa.
"The Houthi movement is a resistance movement. Its political slant is clear: the rejection of the hegemony and global arrogance of the United States."
Emad denied aiming to revive the Zaydi Imamate, or preferring a ruler descended from the Prophet:
"We took part in the revolution (against Saleh) and we have a political vision for a civilian state that assures rights, freedom and equality for all, regardless of which region or tribe people come from."
After a series of wars failed to crush the movement in its north Yemen base of Saada - including one in 2009 during which Saudi Arabia openly joined in against them - the province has fallen openly into Houthi hands with a Houthi-imposed governor.
Houthis clashed with Sunni Islamist Salafis over control of mosques in Amran and Hajjah provinces north of Sanaa in September, and many fear the conflict could move to the capital.
In the al-Qaa quarter, a mosque preacher praises the Zaydi imams.
"He wouldn't dare have said that before, when Saleh was around," muttered Mohammed Bamatraf, a supporter of the security Yemen Socialist Party and an opponent of the former president.
Free to operate, the group sold CDs, books and posters at this year's Sanaa Book Fair and maintains a large tented presence at Change Square, the focus of last year's uprising against Saleh. Along the road, they have daubed the words 'USA' inside blue Stars of David on the tarmac.
Islah, represented in the transitional cabinet, fears the Houthis are trying to bring all northern provinces under their control before entering the national dialogue.
"They are flexing muscles on the streets of the capital... They want a conflict with Salafis in some neighbourhoods," said spokesman Mohammed Qahtan. He said Houthi sectarianism could open a Pandora's box in Yemen.
"We support turning them into a political party, but we don't accept that they replicate Hezbollah in Yemen and become a state within a state."
This may have already happened, analysts say.
"They were able to reach outside their homebase because of corruption and lack of rule of law, though their ideology is not appealing to the majority in Yemen," said political analyst Abdulghani al-Iryani.
"The concept of the Imamate is inbuilt with their ideology but they are not foolish enough to think they can enforce it: that's why they favour decentralisation." (Editing by Sami Aboudi and Sonya Hepinstall)