Russian flags were flying high in the historic naval city of Sevastopol Sunday as people looked forward to rejoining what many see as their homeland after a crunch referendum on Crimea's future.

Several thousand people -- many waving the white, blue and red Russian standard or holding flowers -- gathered for a celebratory concert in the city's Nakhimov Square, named after one of Russia's most illustrious admirals.

Crimea's referendum on joining Russia is "illegal and illegitimate", the European Union said Sunday, announcing that it would decide on sanctions on Monday.

"The referendum is illegal and illegitimate and its outcome will not be recognized," the European Council and European Commission presidents said in a joint statement.

Ukraine's defense minister said Sunday that Russia had agreed to temporarily lift its blockade of Ukraine's military bases in Crimea in order to ease tensions surrounding the peninsula's secession referendum, as Moscow said it would respect the “choice” of the flashpoint region's residents.

"Agreements have already been reached between our commanders... on there being no attempts to blockade our military installations until March 21," Interfax quoted Defense Minister Igor Tenyukh as saying.

Ukraine's former prime minister, Yulia Tymoshenko, urged Western powers Sunday to unite on a hard line against Russian President Vladimir Putin in the Crimea crisis.

"This aggressive striving for power with regard to Ukraine represents not only a danger for the Ukrainian state -- other parts of eastern Europe are also in danger," she told German daily Der Tagesspiegel.

NATO said several of its websites were targeted in a "significant" cyber attack on Saturday that was claimed by Ukrainian hackers in what appeared to be the latest bout of virtual warfare linked to the country's crisis.

Spokeswoman for the military alliance Oana Lungescu said on Twitter that the websites had been hit by "a significant DDoS (denial of service) attack", but that it had had "no operational impact".

The Swiss public ministry is looking into suspicions of money-laundering by deposed Ukrainian president Viktor Yanukovych, a spokeswoman said Saturday.

The information from the Money Laundering Reporting Office Switzerland (MROS) had been reported in the Swiss-German press.

An alternative meeting for the Group of Seven most developed economies could be held in London if Russia is expelled from the G8 for its aggressive policy in Ukraine, a German weekly reported Saturday.

Britain has proposed hosting the summit with Germany, Japan, France, Canada, Italy and the United States, Der Spiegel said, citing an unnamed source close to the German government.

U.S. Senator John McCain called for massive military assistance to Ukraine Saturday, while warning that Russia's actions in its former Soviet neighbor could lead to unprecedented measures by the United States and it allies.

"Ukraine is going to need a long-term military assistance program from the United States," McCain told reporters following a U.S. senate delegation visit to the Ukrainian capital, voicing what he said was a personal opinion.

Russia vetoed a Western-backed resolution condemning the Crimea referendum at a U.N. Security Council emergency vote Saturday but China abstained, isolating Moscow further on the Ukraine crisis.

The draft resolution, which says Sunday's referendum would have no validity, got 13 votes in the 15-member council. But it was rejected when permanent member Russia exercised its veto.

Thousands of people rallied in central Moscow Saturday in protest at Russia's intervention in Ukraine, a day before the Crimean peninsula is expected to vote on switching to Kremlin rule.

Waving Ukrainian flags and shouting slogans heard during the Maidan protests in Kiev, the demonstrators urged Russian President Vladimir Putin to pull troops back.