Yerevan (AFP) - Armenian president Serzh Sarkisian on Monday called for "radical changes" in the country after a tense stand-off with 20 anti-government gunmen holed up in a police station left two officers dead and triggered mass protests.

"One thing is clear, the process of the radical changes in Armenia's social and political life" must be sped up, Sarkisian said at a meeting with civil society representatives, religious leaders and government officials, his office said.

"Yes, it is true that the Armenian authorities are not perfect. Yes, it is true that there are many problems and complex issues in Armenia. Our goal is to give them a speedy resolution."

The 20 pro-opposition gunmen surrendered Sunday after seizing a police station in Armenia's capital Yerevan on July 17.

The crisis, which saw the gunmen take police officers and medical personnel hostage, sparked demonstrations by opposition supporters in Yerevan that sometimes descended into violence.

The gunmen were demanding the release of jailed opposition leader Zhirair Sefilyan and the resignation of Sarkisian, a former communist party leader who came to power in 2008.

Sefilyan is a fierce government critic who was arrested along with six supporters in June. They were accused of preparing to seize government buildings and telecoms facilities.

A statement published on Sunday by one of the gunmen inside the police station vowed the group would continue its fight from prison.

More than 1,000 people took to the streets in downtown Yerevan on Monday night in an anti-government protest calling for leniency toward the armed group, according to local media.

- 'Not Beirut or Aleppo' -

The gunmen plunged Armenia into turmoil when they stormed the police station, killing an officer and taking several others hostage.

They let the officers go but then seized four medical workers before freeing them too.

On Saturday a second officer was killed by a sniper who authorities said was firing from inside the police compound, a claim denied by the gunmen.

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Sarkisian vowed that the country would not allow anyone to "undermine the foundation of our state."

"Problems in Armenia will not be solved through violence or arms," he said. "Yerevan is neither Beirut nor Aleppo."

Sarkisian also called for a "thorough investigation, a comprehensive and unbiased examination and an open trial" over the incident.

Human Rights Watch accused the Armenian police of using "excessive force against peaceful protesters" and said it had "assaulted journalists reporting on the demonstrations" on Friday.

Giorgi Gogia, South Caucasus director at HRW, said Armenia's investigation into Friday's assaults on demonstrators "should be swift and thorough."