Sanaa - Houthi rebels are battling soldiers near Yemen's presidential palace on Monday morning, firing from rooftops as bodies lay in the streets, witnesses said.
The status of President Abed Rabbo Mansour Hadi was not immediately clear.
Witnesses nearby told The Associated Press that heavy machine gun fire could be heard as mortars fell around the palace. Civilians in the area fled the fighting as columns of black smoke rose over the palace.
The fighting caused a number of casualties as ambulance sirens wailed throughout the capital, Sanaa.
"Oh God! There are bodies on street," well-known Yemeni activist Hisham Al-Omeisy wrote on Twitter.
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The Houthis' al-Maseera satellite television channel aired a report accusing the army of opening fire without reason on a militia patrol in the area of the presidential palace, sparking the violence.
A Yemeni military official, speaking to AP on condition of anonymity as he wasn't authorised to brief journalists, said the Houthis provoked the attack by approaching military positions in the area and setting up their own checkpoints.
Schools closed, families trapped
Hadi doesn't live at the palace, but his home nearby was quickly surrounded by additional soldiers and tanks. Schools also closed throughout Sanaa as Houthi rebels manned checkpoints throughout the city. Many families remained trapped in their homes.
"People are leaving on foot, searching for safety," resident Tarfa al-Moamani said.
There was no word on state media about the violence as Sanaa was suffering a power outage at the time. Those able to watch Yemen state television saw a pre-recorded musical performance.
The Houthis seized large areas of Yemen, including Sanaa, last year as part of their protracted power struggle with Hadi. Critics say the Houthis are a proxy for Shiite Iran, charges the rebels deny.
Hadi took over the presidency in 2012 after a popular revolt toppled his predecessor, Ali Abdullah Saleh. In recent days, the Houthis kidnapped Hadi's chief of staff, widening the conflict. In a statement, the Houthis said at the time they abducted the man to disrupt a meeting scheduled for the same day that was to work on a new constitution and the reorganisation of the country into federally organised regions.
Saleh himself is a Houthi, a group that belongs to the Zaydi branch of Shiite Islam that exists almost solely in Yemen. Houthis represent about 30% of Yemen's population.
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Many believe Saleh has been orchestrating the recent Houthi rebel offensive around the country. The United Nations last year put Saleh on a sanctions list, along with two Shiite leaders, for destabilising the country.
Security officials, speaking on condition of anonymity to discuss intelligence matters, said they believed tribal fighters loyal to Saleh were racing into Sanaa to back the Houthis in the fighting.
Monday's battle comes a day after Hadi chaired a meeting in which he demanded the army defend Sanaa, the official Saba news agency reported.
It wasn't clear whether Hadi, who has made similar calls in the past, was issuing a new order for security services to take back control of the city from the Houthis.
Yemen, the Arab world's poorest country, has suffered years of turmoil since the Arab Spring. It also is home to al-Qaeda in the Arabian Peninsula, considered by the US to be the most dangerous arm of the terror group.
That group has said it directed the recent attack against the French satirical magazine Charlie Hebdo in Paris "as revenge for the honour" of Islam's Prophet Muhammad.
The US has carried out a campaign of drone strikes in the country targeting suspected militants. Civilian casualties from those strikes have angered Yemenis.