Ukrainian security forces have detained 70 people in an "anti-terrorist" organization in Kharkiv, the country's Interior Minister Arsen Avakov announced Tuesday.

They were arrested over "illegal actions involving separatism, stirring mass riots, causing corporeal damages and other offences", according to a press release published on the Interior Ministry's website, as quoted by UNIAN news agency.

Avakov wrote on Facebook that the city center of Kharkiv was "blocked along with metro stations", but urged calm as they would be opened again after the operation was over.

Kiev officials reported actions of security forces were coordinated by interim President Oleksandr Turchynov, who on Monday accused Russia of playing out "Crimea scenario" in Eastern Ukraine.

On Tuesday evening, Kharkiv People's Republic was proclaimed by pro-Russian activists following similar declarations in Donetsk earlier that day. Protesters said they did not trust members Kharkiv's regional council and will also seek to hold a referendum on joining Russia. These statements were followed by clashes between pro-Russian and pro-Ukrainian demonstrators in front of the regional assembly building.

Ukrainian special forces also managed to re-take control security service headquarters in Donetsk, which had been seized by activists Monday morning. Donetsk Governor Serhiy Taruta was meanwhile quoted as saying that developments in the region were aimed at "splitting the country".

Prime Minister Arseniy Yatsenyuk earlier headed to Donetsk for crisis talks with local authorities. Former PM and May 25 presidential candidate Yulia Tymoshenko is also known to be in the city.

Interior Minister Avakov also announced that special militia units would be deployed across eastern and southern parts of Ukraine.

Russia responded to the decision by warning Ukraine against "military preparations" which might carry the risk of "sparking a civil war" in the country.

Russia's Foreign Ministry also argued that members of Ukraine's National Guard and of the far-right group Right Sector were being mobilized to the east, "including in Donetsk".

On its website, the Foreign Minsitry argues that 150 US members of the "private military organization Greystone" are involved in the operation.

Moscow repeated its calls for constitutional amendments envisaging federalization in Ukraine, US State Secretary John Kerry was told by his counterpart Sergei Lavrov in a phone conversation on Monday night.

White House Spokesman Jay Carney was quoted by Al Jazeera as saying that the US was ready to impose further sanctions to Russia in case of renewed escalation.

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