There are dozens of unidentified gunmen outside Sloviansk police station, as the BBC's David Stern reports. Photo: BBC

Police said the men fired shots and used stun grenades to seize the offices in Sloviansk, near the Russian border.

The interior minister called the gunmen "terrorists" and said special forces would repel the attack.

Pro-Russian activists have seized government buildings elsewhere in east Ukraine. Kiev accuses Moscow of orchestrating the unrest.

Interim Foreign Minister Andrei Deshchytsia urged Moscow to end "provocative" actions by its agents.

Tensions are rising in the east, as pro-Russian activists continue a stand-off with Ukraine's new government

Eastern Ukraine has a large Russian-speaking population and has seen a series of protests since the ousting of President Viktor Yanukovych in February.

Protesters in largely Russian-speaking Donetsk, 130km (80 miles) from Sloviansk, have been occupying government buildings for days and demanding a referendum on becoming part of Russia.

A similar move prompted a Russian takeover of Ukraine's Crimea region earlier this year.

The US and EU have put sanctions on Russian and Crimean people they say were connected with the takeover.

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At the scene

David SternBBC News, Sloviansk

We did see that the (police) station is taken over, there are barricades out in front and lots of crowds. We're told that there are dozens of unidentified gunmen in green unmarked camouflage uniforms who are moving around there.

We had to leave the town because we were told foreign journalists could come under threat.

It is difficult to say exactly how many people are there and who they are. They have not identified themselves.

We were told by a local journalist that there were a number of people from the town among them but she also said there were also a lot of people who were new to the town - they didn't even know who the mayor was.

We have spoken to one of the spokespeople for the activists in Donetsk, the regional capital - where the government building has been taken over - and he told us that the group in Sloviansk was working with them.

So obviously it is potentially a very volatile situation, especially with the interior minister saying they are going to deal with this takeover harshly.

Russia has denied responsibility for the protests in eastern Ukraine, but Western nations have expressed concern over a build-up of Russian troops along the border.

Interim Prime Minister Arseniy Yatsenyuk visited Donetsk on Friday and attempted to placate anti-government groups by guaranteeing that no restrictions would be put on the use of the Russian language.

The Kiev government had set a deadline of Friday for all occupations to end, but trouble continued in several cities in the east.

Meanwhile, regional police spokesman Ihor Dyomin described how armed men were bussed to the police station in Sloviansk.

"Six or seven unknown persons got out. They fired several shots in the air and attempted to storm the police department," he said.

He added that "people in camouflage uniform" and with weapons" were inside the building.

Interim Interior Minister Arsen Avakov promised to deal with the attackers.

"The response will be very tough because there is a difference between protesters and terrorists," he said in Ukrainian on hisFacebook page.

The Interior Ministry said the aim of the attack was to seize arms from the police station, where some 40 automatic rifles and 400 pistols were stored along with ammunition.

One of those who seized the police station said the group had gone to Sloviansk to fight people who represented the illegal authorities of interim President Oleksandr Turchynov, according to a video posted on Youtube.

Interviewed in Russian, the man said they represented the People's Front of Donbass - the industrial Donets Basin region in eastern Ukraine.

He said those involved were peaceful Donbass residents and "not fascists".

The BBC's David Stern, who was briefly in Sloviansk on Saturday, said from what he saw the take-over was well-organised and the gunmen were quickly spreading their control throughout the town and beyond.

The roadblocks were being reinforced with more tyres as he left, and the people manning them were well-armed, he said.

The occupation by pro-Russian groups of the local government building in Donetsk is meanwhile continuing.

Alexander Gnezdilov, the protesters' unofficial spokesperson, told the BBC the group that seized Sloviansk police HQ was "an independent group who are supporting the Donetsk protest".

Also on Saturday, hundreds of pro-Russian protesters marched towards a police station in the regional capital Donetsk.

They threatened to storm the building, demanding the resignation of city police chief Kostyantyn Pozhydayev.

He later told the protesters he was standing down.

The men in Sloviansk were said to be allied to pro-Russian activists in other cities

Protesters defied a government deadline, continuing a series of blockades and sit-ins

On Saturday morning, a group of men briefly occupied the prosecutor's office in Donetsk