Yerevan (AFP) - A group of hostage-takers locked in a protracted siege with police in Armenia shot dead an officer Saturday, as thousands of protesters took to the streets to call on President Serzh Sarkisian to step down.

"A sniper opened fire from inside the police station and killed a police officer... who was sitting in a car parked 350-400 metres (yards) away," police spokesman Ashot Aharonyan wrote on Facebook.

Armenia has been in turmoil since a group of anti-government gunmen stormed a police station in the capital Yerevan on July 17, demanding the release of opposition leader Zhirair Sefilyan and taking several hostages.

They are currently still holding two medics.

Tensions spiked again on Saturday after security services gave the group a 5:00 pm (1300 GMT) deadline to surrender following overnight clashes with scores of supporters in the city, resulting in dozens of injuries and arrests.

Hours after the deadline passed, some 5,000 people turned out in central Yerevan in support of the pro-opposition gunmen, according to an AFP estimate.

"Bring your relatives and your neighbours onto the streets!" cried protester Albert Bagdassian.

"Our goal is to support the group against which the security services have decided to launch an assault, to march on the street, to paralyse traffic and to show that we are not afraid."

The latest protest came after the authorities used stun grenades, truncheons and smoke bombs to break up a rally late Friday near the police station where the gunmen are holed up.

More than 70 people had to be taken to various hospitals to be treated for injuries including burns and broken limbs.

Journalists were among those hurt.

"Out of 73 injured people, 26 are still in hospital, including six policemen," health ministry spokeswoman Anahit Haytayan wrote on Facebook.

Earlier Friday, police had exchanged fire with the gunmen, wounding two, who were taken to hospital under armed guard.

- Protesters detained -

Armenian police told AFP that 165 people were detained in total during the overnight unrest, most of whom were later released.

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Authorities said they had launched a criminal probe into 23 of the protesters, including a member of the pro-Western Heritage party Armen Martirosyan.

The gunmen -- supporters of fringe jailed opposition leader Sefilyan -- took the police building nearly two weeks ago, killing one officer, taking several more hostage and seizing a store of weapons.

They have since freed all the policemen but on Wednesday seized four medical staff who had entered the compound to treat some of their wounds, two of whom were later released.

The group has demanded the resignation of Sarkisian as well as Sefilyan's release and protesters have regularly gathered since, voicing similar calls.

Sefilyan and six of his supporters were arrested in June, accused of preparing to seize government buildings and telecoms facilities.

The hostage crisis and violence has shaken the small landlocked ex-Soviet nation, just months after a surge in conflict with Azerbaijan over separatist ethnic-Armenian region of Nagorny-Karabakh left 110 people dead in April.

- 'Very worrying' -

The US embassy in Yerevan said in a statement it was "deeply concerned by the shocking images and credible reports of violence and excessive use of force by the police to disperse protesters".

"We urge the Armenian government to take immediate steps to prevent a repeat of last night's actions," the embassy added.

The European Union called the latest developments in the crisis "very worrying".

"Use of force and violence to achieve political change are not acceptable," an EU spokeswoman said in a statement.