Pro-Russian activists in eastern Ukraine have declared Donetsk a sovereign republic and asked Russian President Vladimir Putin to send in peacekeepers, local media reported.

The announcement was made by a republican council of the "Peoples' Republic of Donetsk," who gathered at the local administration building, which was occupied by pro-Russian activists Sunday. They announced that a referendum on whether to join Russia will be held on May 11.

Earlier Monday, Ukrainian Prime Minister Arseny Yatseniuk said Monday that protests in eastern Ukraine in which pro-Russian activists seized public buildings in three cities are part of a plan to destabilize the country and bring in Russian troops.

Saying Russian troops were within a 30-kilometer (19 mile) zone from the Ukrainian border, Yatseniuk told a government meeting: "An anti-Ukrainian plan is being put into operation ... under which foreign troops will cross the border and seize the territory of the country.

"We will not allow this," he said.

Pro-Russian protesters in the east seized official buildings in three cities - Kharkiv, Luhansk and Donetsk - on Sunday night, demanding that referendums be held on whether to join Russia.

A similar move preceded a Russia-backed takeover of Crimea in March followed by annexation of the peninsula by Russia.

Interior Minister Arsen Avakov said on Monday the main regional administration building in Kharkiv had been cleared of "separatists".

But police said protesters occupying the state security building in Luhansk had seized weapons and highway police had closed off roads into the city.

"Unknown people who are in the building have broken into the building's arsenal and have seized weapons," a police statement said. Nine people had been hurt in the disturbances in Luhansk.

Mainly Russian-speaking eastern Ukraine has seen a sharp rise in tension since Moscow-backed Ukrainian President Viktor Yanukovich's overthrow in February and the advent of an interim government in Kiev that wants closer ties with Europe.

Russia has branded the new leadership in Kiev illegitimate and has annexed Ukraine's Crimea region, citing threats to its Russian-speaking majority - a move that caused the biggest standoff between Moscow and the West since the end of the Cold War.

The protesters appeared to be responding in part to Yanukovich, who fled to Russia after he was ousted and who on March 28 issued a public call for each of Ukraine's regions to hold a referendum on its status inside the country.

Russia has been pushing internationally a plan proposing the "federalization" of Ukraine in which regions of the country of 46 million would have broad powers of autonomy.

Ukraine, while drawing up its own blueprint of constitutional changes for 'de-centralization' in which smaller municipalities would be able to develop their own areas by retaining a portion of state taxes raised, says the Russian plan is aimed at breaking up the country.

Yatseniuk said that though much of the unrest had died down in eastern Ukraine in the past month there remained about 1,500 "radicals" in each region who spoke with "clear Russian accents" and whose activity was being coordinated through foreign intelligence services.

But he said Ukrainian authorities had drawn up a plan to handle the crisis.

"We have a clear action plan," he said, adding that senior officials would head to the towns concerned.

Referring to the Russian plan, Yatseniuk said: "It is an attempt to destroy Ukrainian statehood, a script which has been written in the Russian Federation, the aim of which is to divide and destroy Ukraine and turn part of Ukraine into a slave territory under the dictatorship of Russia," he said.

"This is not going to happen," he said.

"I appeal to the people and the elites of the east. Our common responsibility is to preserve the country and I am sure that no-one wants to be under a neighboring country. We have our country. Let's keep it," he said.