Yemeni police killed at least seven Shia Houthi activists in the capital Sanaa when they opened fire on hundreds marching on government headquarters, a protest organiser has told Al Jazeera.

Al Jazeera's Hashem Ahelbarra, reporting from Sanaa on Tuesday, said there was a "serious escalation" of violence under way.

"More than 50 people were also injured in the clashes. There has also been reports of fighting on the outskirts of the city, which is a very dangerous development."

Houthi activists say they were merely protesting and marching to the office, but the interior ministry said the rebels were en route to break into government buildings.

Yemen's supreme security committee, headed by President Abd-Rabbu Mansour Hadi, said however, it held the Houthi rebels responsible for the deaths, stating that gunmen among the protesters had opened fire and not the guards. 

Meanwhile, Yemeni troops and Shia rebels exchanged gunfire at the western entrance to Sanaa. There were no immediate reports of injuries in the clashes.

"The situation is now very delicate in the capital," our correspondent said.  

The rebel activists have been camped out in Sanaa for weeks in a campaign to bring down the government and reverse unpopular fuel price increases.

Fighting has also intensified in the northern al-Jawf province where the government is conducting air strikes against Houthi positions.

The authorities claim a senior rebel commander was killed in one of these raids.

The Houthis are seeking to oust the government and reverse unpopular fuel price rises, but officials say they
have a sectarian agenda to carve out their own semi-independent state and gain a veto power in national politics. 

Source: Al Jazeera and agencies