Troops are seen at the main square in Almaty, Kazakhstan, on Thursday. Reuters

Troops and protesters clashed in Kazakhstan's largest city Almaty on Thursday, with police saying tens of rioters had been "eliminated" as they tried to storm their offices.

In the worst reported violence so far, police said dozens of people were killed in battles with security forces at government buildings in the country's largest city Almaty.

Meanwhile, a Moscow-led military alliance said on Thursday it had sent its first troops to Kazakhstan after its government requested help to quell mounting unrest.

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"Peacekeeping forces of the Collective Security Treaty Organisation were sent to the Republic of Kazakhstan for a limited time to stabilise and normalise the situation," the CSTO secretariat said in a statement posted online by Russian foreign ministry spokeswoman Maria Zakharova.

"Last night, extremist forces tried to assault administrative buildings, the Almaty city police department, as well as local police commissariats. Dozens of assailants were eliminated," police spokesman Saltanat Azirbek was quoted as saying by Interfax-Kazakhstan, TASS and Ria Novosti news agencies.

Kazakhstan's President Kassym-Jomart Tokayev speaks during his televised statement on Wednesday. AP

Several armoured personnel carriers and dozens of troops entered the main square of Almaty on Thursday morning where hundreds of people were protesting against the government for the third day, Reuters correspondents reported from the scene.

Gunshots were heard as troops approached the crowd, according to Reuters witnesses, but the situation in the square had calmed down since then.

Around Kazakhstan, protests initially sparked by a fuel price rise killed eight police and national guard troops on Tuesday and Wednesday, prompting Kazakh president to appeal for help from a Russia-led security alliance, which offered to send peacekeeping forces.

A view shows a burning police car during a protest in Almaty, Kazakhstan, on Wednesday. Reuters

State television reported on Thursday that the National Bank of Kazakhstan had decided to suspend work of banks in the country for the safety of their workers. The Internet in the country is mostly down.

Videos on social media on Thursday showed pillaged shops and burned buildings in Almaty, automatic gunfire in the streets and residents screaming in fear.

The growing unrest is presenting a major challenge to the authoritarian regime of President Kassym-Jomart Tokayev and has raised international concern, especially with key ally Moscow.

'Outside interference'

With his repeated calls for calm ignored, Tokayev in an address to the nation early Thursday said he had appealed to the Russia-led Collective Security Treaty Organisation (CSTO), which includes five other ex-Soviet states, to combat what he called "terrorist groups" that had "received extensive training abroad".

The CSTO's chairman, Armenian Prime Minister Nikol Pashinyan, said the alliance would send "collective peacekeeping forces... for a limited period of time in order to stabilise and normalise the situation in this country" that was caused by "outside interference".

Protesters take part in a rally over a hike in energy prices in Almaty on Wednesday. AFP

There were no immediate further details nor comment from Moscow, which dominates the alliance.

Tokayev said "terrorists" were seizing buildings, infrastructure and small arms, and fighting battles with security forces.

Protests spread across the nation of 19 million this week in outrage over a New Year increase in prices for liquid petroleum gas (LPG), which is widely used to fuel cars in the west of the country.

Thousands took to the streets in Almaty and in the western province of Mangystau, saying the price rise was unfair given oil and gas exporter Kazakhstan's vast energy reserves.

Protesters were reported to have stormed several government buildings on Wednesday, including the Almaty mayor's office and the presidential residence, with both said to be on fire.

As of late Wednesday, at least eight law enforcement officers had been killed and 317 wounded in the violence, according to the interior ministry quoted by local media.

The full picture of the chaos was unclear, with widespread disruptions to communications including mobile phone signals, the blocking of online messengers and hours-long internet shutdowns.

The protests are the biggest threat so far to the regime established by Kazakhstan's founding president Nursultan Nazarbayev, who stepped down in 2019 and hand-picked Tokayev as his successor.

Kazakhstan's former president Nursultan Nazarbayev (right) attends a meeting. File/AFP

Tokayev tried to head off further unrest by announcing the resignation of the government headed by Prime Minister Askar Mamin early on Wednesday, but protests continued.

State of emergency

Tokayev also announced he was taking over from Nazarbayev as head of the powerful security council, a surprise move given the ex-president's continued influence.

With protests escalating, the government late on Wednesday said a state of emergency declared in protest-hit areas would be extended nationwide and in effect until January 19.

It imposes an overnight curfew, restricts movements and bans mass gatherings.

Riot police officers patrol a street in Almaty on Wednesday. AFP

Much of the anger appeared directed at Nazarbayev, who is 81 and had ruled Kazakhstan since 1989 before handing power to Tokayev.

Many protesters shouted "Old Man Out!" in reference to Nazarbayev and images posted on social media showed a statue of the ex-president being torn down.

The EU and the UN called for "restraint" on all sides, while Washington urged authorities to allow protesters to "express themselves peacefully."

Kazakhstan's government tolerates little real opposition and has been accused of silencing independent voices.

Spontaneous, unsanctioned protests are illegal despite a 2020 law that eased some restrictions on freedom of assembly.