Ukraine has slipped deeper into crisis ahead of tomorrow's separatist referendum in eastern regions, with deadly clashes in the city of Mariupol and a first visit from Russian leader Vladimir Putin to annexed Crimea.

In parts of Donetsk and Luhansk regions where they occupy official buildings, pro-Moscow rebels are preparing for a plebiscite on splitting from Ukraine, defying doubts over whether they can hold a credible vote and rejection of its legitimacy by Kiev and the West.

Fear and tension stoked by sporadic clashes between security forces and militants across the two provinces soared yesterday, when fierce fighting erupted in the city of Mariupol and armoured vehicles roared through the streets.

Footage from the port on the Sea of Azov showed soldiers exchanging fire with rebel gunmen, and armoured vehicles flying the Ukrainian flag being surrounded by furious locals who, like many easterners, reject the authority of Kiev’s new pro-EU government.

"A group of about 60 terrorists with automatic weapons attacked the city police station," said interior minister Arsen Avakov.

“As a result of the battle, which involved the national guard, the military and the special ‘Azov’ unit of the interior ministry . . . about 20 terrorists were destroyed and four captured,” Mr Avakov said, adding that at least one member of government forces was killed and five hurt.

Casualty figures
The figures could not be confirmed, and the militants did not immediately release casualty numbers. Health officials said three people died and 25 were injured.

Locals accused the security forces of firing heavy weapons at the police station and shooting at unarmed people, including a Russian journalist who was reportedly wounded.

The police building was left gutted when the military withdrew from central Mariupol yesterday afternoon, and anti-Kiev protesters were seen with a captured armoured vehicle that some reports said had been abandoned due to engine trouble.

Mr Avakov said the “anti-terrorist” crackdown in the east – which has been a halting, sometimes hapless operation – was getting tougher with the introduction of reinforcements from regions where support for the government is much stronger.

“For those who come with weapons, shoot, take hostages, torture them, rob people and cover themselves with various slogans, there will be only one reply from the Ukrainian state – destruction,” Mr Avakov said.

A group of armed fighters from the city of Donetsk, about 100km north of Mariupol, said they were travelling to the port city to support local rebels.

They were given a send-off by thousands of people who gathered on Donetsk’s Lenin Square to celebrate Victory Day, which marks the Soviet defeat of fascist Germany.

Many of them marched to the square with second World War veterans and leaders of the separatist movement, who have taken over the regional administration building and declared it the headquarters of their “Donetsk People’s Republic”.

To chants from the crowd of "Russia, Russia" and "Down with the junta" – their term for the Kiev government – rebel activists took to the stage to urge people to vote for Donetsk's independence from Ukraine in tomorrow's hastily arranged referendum.

“Glory to our veterans. Without them we wouldn’t be here, our country wouldn’t be here and perhaps Europe wouldn’t be here,” said separatist leader Denis Pushilin.

“No one expected fascism to come back, and to come so close to our homes. But those who wanted to defend their homeland have stood up, and we have already have a small victory – the declaration of our Donetsk People’s Republic,” he said.

He said the next stage would be a vote backing its independence from Ukraine, which many easterners – and Moscow – say is now run by Russian-hating fascists.

Kiev and western nations say such allegations are absurd, and are used by the Kremlin and its allies in Ukraine to whip up fear and provide a pretext for Russian interference in a country that was poised to tilt westwards with the ousting of Moscow-backed president Viktor Yanukovich in February.

Many people at the Donetsk rally said they supported independence because they feared fascism and wanted to retain close ties with Moscow.

Others – echoing speakers onstage – expressed regret at the Soviet Union’s demise and spoke of a desire for a state that would unite ethnic Russians in Russia itself, southern and eastern Ukraine and Belarus.

Flags glorifying the Soviet Union and two of its leaders, Vladimir Lenin and Josef Stalin, flew over the gathering, which took place on Donetsk's Lenin Square.

Red Square display
At about the same time in Moscow, Mr Putin presided over a spectacular – and, for some, ominous — display of military might, as thousands of troops marched across Red Square, followed by tanks and missile launchers and a fly-past by an array of bombers and fighter jets.

“This is a day when we all feel particularly keenly what it means to be faithful to the motherland and how important it is to be able to defend its interests . . . It is a holiday when the all-conquering power of patriotism celebrates victory,” Mr Putin said before flying to Crimea to inspect a similar military show in the Black Sea port of Sevastopol.

He sent troops into Crimea soon after Mr Yanukovich fled for Russia, and quickly annexed the region for Moscow despite outrage from the EU and US, which responded by placing sanctions on politicians and businessmen with links to the Kremlin. Referring to a disputed referendum on the peninsula, Mr Putin said Crimeans had chosen to “return” to Russia, and shown “that they remain true to the historic truth and our forefathers’ memory”.

“We have many difficulties ahead, but we will overcome these difficulties because we are together, and this makes us even stronger,” he told a rapturous crowd.

Maja Kocijancic, spokeswoman for EU foreign policy chief Catherine Ashton, said Brussels "notes with regret" Mr Putin's visit to Sevastopol and use of Victory Day "to give visibility to the illegal annexation of Crimea".

The EU and US have threatened to impose more sanctions on Russia if it continues to “destabilise” Ukraine, and may do so as soon as Monday.