By Darya Korsunskaya and Alexander Winning
SEVASTOPOL (Reuters) - President Vladimir Putin flew in to Crimea on Friday for the first time since it was annexed by Moscow, proclaiming as he marked the Soviet victory in World War Two that incorporating the former Ukrainian territory had made Russia stronger.
In east Ukraine, where pro-Moscow rebels plan a referendum on Sunday to follow Crimea in breaking from Kiev, at least seven people were killed and dozens were wounded in chaotic fighting in the center of the port of Mariupol.
One of the most serious clashes yet between Ukrainian forces and separatists, it edged the former Soviet republic closer to civil war.
The head of NATO, locked in its gravest confrontation with Russia since the Cold War, condemned Putin's visit to Crimea, whose annexation in March has not been recognized by Western powers. He also renewed doubts over an assurance by the Kremlin leader that he had pulled back troops from the Ukrainian border.
The pro-Western government in Kiev, labeled "fascist" by Moscow, said Putin's visit was intended to escalate the crisis.
Watching a military parade in Sevastopol on the Black Sea, Putin said: "There is a lot of work ahead but we will overcome all difficulties because we are together, which means we have become stronger."
Earlier in the day, he had presided over the biggest Victory Day parade in Moscow for years. The passing tanks, aircraft and intercontinental ballistic missiles were a reminder to the world - and Russian voters - of Putin's determination to revive Moscow's global power, 23 years after the Soviet collapse.
"The iron will of the Soviet people, their fearlessness and stamina saved Europe from slavery," Putin said in a speech to the military and war veterans gathered on Red Square.
The United States said the trip to Crimea was provocative, the European Union said Putin should not have used the World War Two commemoration to showcase the annexation and NATO Secretary-General Anders Fogh Rasmussen called the visit "inappropriate".
The head of the U.S.-led defense pact was speaking in formerly Soviet Estonia, one of a host of east European nations that joined after the collapse of communism, seeking refuge from the power of Moscow, which many in the region regarded as having enslaved them following its victory in World War Two.
Ukrainian Prime Minister Arseny Yatseniuk, in office since an uprising overthrew the Kremlin-backed elected president in Kiev in February, rejects Russian allegations that his power is the result of coup backed by neo-Nazi Ukrainian nationalists.
"Sixty-nine years ago, we, together with Russia, fought against fascism and won," he said after a Victory Day church service in the capital. Now, he added, "history is repeating itself but in a different form".
Where Russia and Ukraine stood shoulder to shoulder in the past against Germany, now Germany was "standing shoulder to shoulder with us", along with the United States and Britain.
Ukraine's SBU security service accused Russian saboteurs of setting a fire that briefly disrupted state broadcasts.
In Mariupol, eastern Ukraine's main port on the Sea of Azov, regional authorities said seven people had been killed and 39 wounded in the course of an "anti-terrorist operation" - the term Kiev uses for its fight against the separatists.
Journalist Tetyana Ignatchenko said there was fierce fighting outside the police headquarters and video showed armored cars smashing barricades and soldiers exchanging fire with a gunman on the street while crowds milled around.
Ukrainian Interior Minister Arsen Avakov said about 20 "terrorists" were killed when pro-Russian militants tried to seize the city's police headquarters. A figure he gave earlier this week of 30 rebel dead in another city was not confirmed.
A member of Ukraine's parliament, Oleh Lyashko, gave a different account of Friday's events, saying eight rebels had been killed in clashes when Ukrainian forces attacked Mariupol's police headquarters to try to drive out pro-Russian militants.
A local photographer in Mariupol told Reuters the building was ablaze and that two bodies were lying in the street outside.
"One of them is definitely a police officer," he said.
Ukrainian forces later withdrew from the town, a major industrial center with a population of about half a million.
Russian Foreign Minister Sergei Lavrov said after a telephone call with U.S. Secretary of State John Kerry on Friday that Moscow hoped Washington would work with Kiev to end Ukraine's military operation against the separatists.
In Sevastopol, where Russia's Black Sea Fleet previously had to lease its base from Ukraine, servicemen and veterans marched in a parade before Putin's arrival that also included armored vehicles and anti-aircraft missiles. Banners read "Sevastopol without Fascists" and "It's our duty to remember".
"I'm here to prevent any provocations from the fascists. I served in a self-defense unit during March, and I consider it my duty to be here," said Natalya Malyarchuk, 52.
The ethnic Russian majority among Crimea's two million population broadly welcomed the Russian takeover that came in the wake of the Kiev uprising. Given by Soviet leaders to Ukraine only in the 1950s, many Russians long saw it as rightfully theirs. Western powers have imposed sanctions against Russia in response, but reactions have been muted.
Moscow says it has no direct control over the armed rebels in eastern Ukraine running Sunday's referendum on secession from Kiev for the mainly Russian-speaking region.
The European Union is likely to strengthen its targeted sanctions against Russia on Monday, Janusz Lewandowski, a member of the EU's governing commission, said on Friday. Diplomats said they would target about 15 people and several Crimean branches of Ukrainian companies taken over by Russians.
While Putin's redrawing of European borders has sparked great alarm across the continent, U.S. and European leaders are concerned not to harm their own economies by isolating Moscow.
And there is little popular support in the West for an armed conflict with Russia on behalf of Ukraine, a country that is not a member of NATO and where successive leaders have left a legacy of corruption, poverty and feeble state institutions.
In Sevastopol, factory worker Vasily Topol, 31, wearing a white T-shirt with an image of Putin in sunglasses and the words "Russia's Army", said life was better since Crimea became Russian.
"We have the greatest admiration for Putin, we are morally and materially better off since Crimea became part of Russia," he said, speaking on an embankment overlooking Russian warships.
In Slaviansk, the military stronghold of the separatists in eastern Ukraine, separatist "people's mayor" Vyacheslav Ponomaryov and a guard of militiamen led a march of around 2,000 people to lay flowers at a memorial to the World War Two dead.
Veteran Anatoly Strizhakov said: "Look at all these people - the children, the women, the pensioners... Today shows we've got the spirit to stand up to whatever the Ukrainians are planning."
Ponomaryov, who fired a pistol three times in the air during the ceremony, reassured people it would be safe to vote on Sunday. Voters in the two regions, with a combined population of over 6 million, will be asked to vote Yes to the secession of self-styled "People's Republics" in Donetsk and Luhansk.
Opinion polls in recent months have indicated that support for such a move is far from solid and it is unclear how many people will actually take part in voting. A referendum in Crimea in March, which many boycotted, backed secession by 97 percent.
(Additional reporting by Nigel Stephenson and Katya Golubkova in Moscow, David Mardiste in Tallinn, Ralph Boulton, Pavel Polityuk, Aleksandar Vasovic and Elizabeth Piper in Kiev and Alessandra Prentice in Slaviansk; Writing by Alastair Macdonald and Philippa Fletcher; Editing by Giles Elgood)