DONETSK, Ukraine (AFP) - Tension was running at fever pitch in eastern Ukraine on Saturday on the eve of an independence referendum, as rebels briefly held several Red Cross staff on suspicion of espionage.

While preparations were in full swing for the disputed vote in the two eastern regions of Donetsk and Lugansk, the port city of Mariupol to the south observed a day of mourning for at least 21 killed in violent clashes on Friday.

The West believes the referendum could lead to the two regions asking to join Russia and has stressed that holding the vote will deepen what is already the worst crisis in relations with Moscow since the end of the Cold War.

The International Committee of the Red Cross in Geneva said nine of its staff - four from Donetsk, four from Kiev and one from Switzerland - were held "for a few hours" before being released apparently unharmed.

"What we know is that no one was injured," Mr David Pierre Marquet, ICRC spokesman for Europe and Central Asia, told AFP. "They were there to evaluate the needs of the population in Donetsk. That is probably why they were freed so quickly, their mandate was very clear," he added.

Earlier on Saturday, Mr Kiril Rudenko, deputy spokesman for the self-proclaimed "People's Republic of Donetsk", said the Red Cross workers had been detained because the rebels thought they were spies. "They were arrested yesterday. We suspect they were engaged in espionage and we are currently checking these accusations," said Mr Rudenko.

The brief detention underscored jitters in the two regions of east Ukraine ahead of the disputed referendum likely to result in a new spike of geopolitical tension. Rebels in the two regions - with a total population of 7.3 million, out of 46 million for all of Ukraine - defied Russian President Vladimir Putin's call to postpone the vote to allow for a dialogue to dial down the crisis.

In a sudden about-face on Wednesday that stunned the world, Mr Putin also backed planned presidential elections in Ukraine on May 25 but only if Kiev stopped its campaign to flush out separatists from some dozen towns and cities they control in the region.

However, there seemed little sign of an end to the bloodshed that has pushed Ukraine to the brink of civil war as violence erupted in the southern port city of Mariupol on Friday, claiming at least 21 lives.

Interior Minister Arsen Avakov said on his official Facebook page that the chief of the city's police force had also been captured and that snipers had been active during the violence that occurred as Ukraine commemorated the Soviet victory over Nazi Germany in World War II.

An attempt by around 60 rebels armed with automatic weapons to storm the city's police headquarters turned into a "full-scale military clash" when army and interior ministry troop reinforcements arrived, Mr Avakov said. He put the death toll from the near-two-hour combat at 20 rebels and one policeman, while another four policemen were wounded and four rebels were captured.

Witnesses in Mariupol told AFP the fighting was ferocious and involved an exchange of automatic gunfire and shelling from eight armoured vehicles.

In Russia, Mr Putin marked Victory Day with a display of patriotic bombast and a parade of military hardware in Red Square, declaring that Russian "patriotic force" was "all-conquering". He later flew to Crimea, which Russia annexed in March, to inspect the Black Sea fleet - a visit that drew sharp rebukes from the West and Kiev.

Ukrainian authorities said the visit was a "flagrant violation" of the country's sovereignty and voiced its "strong protest" of the "unapproved" trip. The White House also condemned the trip, with National Security Council spokesman Laura Magnuson saying it would "only serve to fuel tensions".

Nato head Anders Fogh Rasmussen criticised the visit as "inappropriate" given Crimea's "illegal, illegitimate" annexation.

Voters in Sunday's referendum will first be asked if they support the creation of an independent "People's Republic" that many see as a prelude to joining Russia, as happened in Crimea. The head of Donetsk's separatists, Denis Pushilin, has said the referendum will be held in "90 per cent of the towns in the region" and turnout is expected to be 60 per cent.

A poll released on Thursday by the Pew Research Centre in the United States suggested 70 per cent of Ukrainians in the east want to stay in a united country, while only 18 per cent back secession. Two in three respondents in the east, however, are unhappy with the Western-backed government in Kiev.

The referendum comes after a recent spike in violence has killed more than 100 people. In addition to the 21 dead in Mariupol, some 14 troops have been killed and 66 servicemen wounded in Ukrainian army assaults on the rebels. The fighting has also claimed the lives of more than 30 insurgents. Clashes that resulted in a horrific inferno in the southern port city of Odessa last week claimed another 42 lives, most of them pro-Russian activists.