Last September, thousands of fighters from northern Yemen seized control of the country's capital, Sanaa. The government was weak, its army fractured, and the rebels - called the Houthis - took the city in only four days.

The secretive Houthi movement was always a mystery to me.

I went to Yemen to follow them, to understand where they came from and what they want since they have suddenly become the most powerful people in Yemen.

I discovered a divided country. The Houthis who belong to the Zaidi sect- an offshoot of Shia Islam, still control the capital, but face a determined alliance of al-Qaeda and other Sunni militants further south.

Mass protests against the Houthis have been reported in some of Yemen's largest cities. I encountered a very different mood - and a sense of the country fragmenting - as I crossed front lines and travelled the country speaking to the Houthis and their enemies.

SANAA

During my first week in Sanaa, al-Qaeda bombs the main square, as the Houthis, in power for only a few weeks, are staging a rally.

"The power of the explosion threw people in the air," a witness tells me when I arrive on the scene soon afterwards. "There were many dead children and old men. So many people." A suicide bomber is blamed for the carnage.

The Houthi slogan is posted on walls across Sanaa. It's an Iranian-inspired political chant from the days of that country's 1979 revolution and reads: "God is great. Death to America. Death to Israel. God curse the Jews. Victory for Islam."

They have established a so-called Revolutionary Committee, now the de facto government, which claims to be clamping down on corruption.

While in Sanaa, I stay with the family of my close friend Radiya, a human rights activist, and her father Dr Mohamed Al Mutawakil, a politician.

"Honestly, I think this is the worst phase Yemen has ever gone through," Radiya tells me.

When I return to the city a few weeks later, the mood has changed. Houthi slogans are crossed out everywhere.

There are rumours that the Houthis are targeting Sunni mosques in the capital and changing their imams. At one mosque I see many signs saying "the Houthis are not the people" and more crossed-out slogans. The call to prayer has been changed from the Sunni to the Zaidi.

According to human rights groups, Houthis are assassinating and torturing their opponents across the city.

Then I hear news of an assassination very close to home. Dr Mohamed Al Mutawakil, whose house I have been staying at while I've been in Sanaa, has been shot and killed.

It's a shock to everyone. He was one of the few men in Yemen who most would agree was a great and honest politician. I am grief-stricken along with the rest of his family and friends.

We suspect al-Qaeda killed him, but no-one claims responsibility.

SAADA

The Yemeni government launched six wars against the Houthis in the last decade - it is a movement born of war.

For years I had tried to go to Saada, but the government rarely allowed journalists in, adding to the region's sense of isolation.

Image caption Saada mosque - birthplace of the Zaidi sect in Yemen

In 2004, the Houthis' leader and namesake, Hussein Badr al-Din al-Houthi ended up hiding from the Yemeni army with his men in caves in the mountains of Marran. Al-Houthi's followers say the attackers poured petrol into the cave and set it on fire, after which he was allegedly captured and killed. But the violence continued.

By 2009, Saudi Arabia had joined the Yemeni army in attacking Saada - the Houthis' stronghold - just across its southern border.

The old city of Saada, not far from the mountains, still bears the scars of this sixth and last war. Buildings have gaping holes - some are just piles of rubble.

Image caption Cemetery in Saada for martyrs of the first war

One of the few women who stayed during the conflict, Um Zayd, tells me she and her family lived under attack for seven or eight months.

"No-one demanded the siege to be lifted. No-one helped us. All this injustice that fell on Saada, we had no fault in it."

Her back yard is now a cemetery - the resting place of three children who were killed by artillery fire. "When we came to them, they were in shreds," Zayd recalls. "Their mother arrived and had to pick up the pieces."

The violence the Houthis endured foreshadowed the treatment they would mete out to their opponents.

RADAA

Al Bayda province used to be the heartland of al-Qaeda in Yemen, but the Houthis now have a presence. There are numerous Houthi checkpoints.

Three weeks before I arrived in the city of Radaa there was an intense battle. Many died on both sides, but the Houthis managed to drive al-Qaeda's soldiers out.

Walid, a Houthi soldier, shows me around.

"In this place here, the people of the town met with the members of al-Qaeda," he tells me, pointing to an open area near a mosque. "We broke bread together in this place, and the next day, they opened fire on our homes with heavy weapons and snipers."

I hear about an al-Qaeda suicide bomber who reached the outskirts of the city, and tried to sneak past a Houthi checkpoint with a small child in the front seat. The car was stopped. It exploded. The driver and children were killed. Now only rubble remains.

The Houthis have destroyed parts of the city too. Walid shows me a pile of semi-pulverised bricks and stone - the home of an extremist he claims.

"He used to kidnap people from Radaa and bring them to this house and torture them," Walid says. "He terrorised the whole town." The Houthis destroyed his home to make an example of him, Walid says. Then he ushers me on.

"OK we are done. One is enough. Otherwise, they will say we destroyed all the houses in Radaa."

BEYOND RADAA

A group of soldiers clad in camouflage and toting guns meets the BBC team as they cross into al-Qaeda territory. Many of the Sunni tribes here are allied with al-Qaeda but not all. They're united in their fight against the Houthis though.

Thousands of Sunni fighters across Bayda province are now united behind a tribal leader, Ziyad Al Majdali.

"They blow up mosques and schools and then scream 'Death to America. Death to Israel,'" he says. Do those mosques belong to America or Israel, he asks.

"If I and an al-Qaeda fighter are fighting in the same trenches against the Houthis, he becomes my brother, my brother in arms," he adds.

In other parts of Bayda, some people say al-Qaeda isn't tough enough. Ahmad Khamis, a prominent local jihadist, says he loves Islamic State.

"IS is a reality and they control land. They will take over districts and will engage in direct battle. They won't retreat from battle, just like in Iraq" he says. "This is our hope to be ruled by Islam and freed from Shia occupation."

ADEN

I head to the largest city in southern Yemen, Aden, to see how other areas outside Houthi control are reacting.

For decades here they have been calling for separation from the north. The Yemeni government has consistently used violence against the secessionist movement in its attempt to crush it.

There is strong anti-Houthi sentiment here. "Death to the Houthis" graffiti dots the city. An enormous crowd gathers for a secession rally. They wave the national flag and cluster around a stage.

"Do you want unity with the north?" a man on stage yells into a microphone.

"No!" the crowd shouts in response.

"We are sick and tired of this," a protester tells me. "The South demands its freedom."

The crowd begins chanting: "Lift your head up high. You are an independent Southerner!"

The rally turns to a march and the people flow into the city's streets - pulsing with passionate anger. Watching this, I can almost feel Yemen collapsing.

I ride through the streets on the back of a pick-up truck, with Abdulrahman, a Houthi journalist. "There is a revolution in the north and a revolution in the south," he says. "The Yemeni people are strangers in their own country."

Suddenly, the sound of gunshots rings out. Security forces are shooting tear gas and live bullets at the protesters.

In all the years I have been coming to Aden, I have witnessed the government's violent crackdown on the protests. This time was no different.

But the rise of the Houthis has polarised the country even more and pushed its people further into deadly conflict.

Since my visit, the South and the East of the country have rejected Houthi control.

The president of Yemen, Abd Rabbuh Mansour Hadi, has fled Sanaa and called for a new government based in Aden.

The Houthis' revolutionary committee has become the de facto government of north Yemen.

Listen to Safa AlAhmad in conversation with Lyse Doucet for this week's Assignment on the BBC World Service, on Thursday 19 March, andafterwards on the BBC iPlayer.

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