Crimea voted to secede from Ukraine in a referendum that most of the world has condemned as illegal. Early results – when 50% of the votes were counted – showed that 95.5% of ballots were in favour of joining Russia.

Russia's lower house of parliament will pass legislation allowing Ukraine's southern Crimea region to join Russia "in the very near future", news agency Interfax cited its deputy speaker as saying on Monday morning.

"Results of the referendum in Crimea clearly showed that residents of Crimea see their future only as part of Russia," Sergei Neverov was quoted as saying.

As the results rolled in, they were met with neither surprise nor welcome by the west. Russian president Vladimir Putin told Barack Obama in a phone call on Sunday night that the referendum endorsing Crimea becoming part of Russia was legal and should be accepted, according to the Kremlin. However, Obama said that the US rejected the results and warned that Washington was ready to impose sanctions on Moscow over the crisis.

The White House said that Obama "emphasised that Russia's actions were in violation of Ukraine's sovereignty and territorial integrity and that, in co-ordination with our European partners, we are prepared to impose additional costs on Russia for its actions".

People in Crimea vote in the referendum on whether to join the Russian Federation or remain as an autonomous republic in Ukraine. Photograph: Zuma/REX

Obama told Putin the crisis could still be resolved diplomatically, but said the Russian military would need to first stop its "incursions" into Ukraine, the White House said.

Putin told Obama the vote was "fully consistent with the norms of international law and the UN charter", according to a statement on the Kremlin website.

The European Union also condemned the referendum as illegal and said it is taking steps to increase sanctions against Russia. EU foreign ministers will meet today to decide whether to impose asset freeze and visa sanctions and, if so, who to target.

"The referendum is illegal and illegitimate and its outcome will not be recognised," Herman Van Rompuy, president of the European council, and José Manuel Barroso, European commission president, said in a joint statement on Sunday.

Artists of the Russian Black Sea fleet dance and music groups perform at pro-Russian celebrations in Lenin Square, Simferopol. Photograph: Max Vetrov/AP

France and Germany echoed the statement by the British foreign secretary, William Hague, that Moscow must face "economic and political consequences".

Earlier on Sunday, Russia and Ukraine agreed a truce in the region until Friday, Ukraine's acting defence minister announced, in a move that may ease tension between Moscow and the western-backed government in Kiev. Speaking on the sidelines of a cabinet meeting, Ukraine's acting defence minister, Ihor Tenyukh, said the deal has been struck with Russia's Black Sea fleet and the Russian defence ministry. "No measures will be taken against our military facilities in Crimea during that time," he said. "Our military sites are therefore proceeding with a replenishment of reserves."

The agreement has provided some respite for Ukraine's beleaguered troops, who have been trapped on their military bases and naval ships since Russian forces began occupying the peninsula on 27 February. They have been encircled ever since, in some cases without electricity. Local residents have smuggled in food to them amid a nervous standoff with the Russian military.

But there seems little doubt that Ukrainian forces will be evicted from Kremlin-controlled Crimea once the truce expires. Crimea's deputy prime minister, Rustam Temirgaliyev, said on Sunday troops would be given safe passage and predicted that eastern Ukraine would be next to join Russia. "Donetsk, Lugansk, Kharkiv have the same situation as in Crimea – 75% of people want to join Russia in eastern Ukraine," he told journalists near the parliament building in Simferopol.There was further turmoil in Donetsk when pro-Russian protesters stormed the prosecutor's office and removed the Ukrainian flag from the roof raising a Russian flag in its place. Riot police deployed to protect the building made little effort to stop the crowd, which later dispersed.

The government in Kiev has accused Moscow of deliberately stirring up tensions in the east by bringing in professional activists and provocateurs from across the border. In a series of ominous statements, Russia's foreign ministry has said it may be forced to act to "protect" ethnic Russians – an expression that appears to provide a rationale for future military incursions.

Putin spent Sunday evening at the closing ceremony of the Paralympics in Sochi but was keeping an eye on the Crimea results, his spokesman said. Earlier he had expressed concern about the escalation of tensions in the south and south-eastern regions of Ukraine, Reuters reported.

Pro-Russian people celebrate in Lenin Square, in Simferopol, Ukraine. Fireworks exploded and Russian flags fluttered above jubilant crowds after residents in Crimea voted overwhelmingly to secede from Ukraine and join Russia. Photograph: Max Vetrov/AP

He blamed the febrile mood on "radical forces" acting with the "connivance of the current Kiev authorities". The Kremlin refuses to recognise Kiev's temporary government that it says came to power on the back of a "fascist" coup.

Putin telephoned the German chancellor, Angela Merkel, on Sunday and told her the referendum in Crimea, condemned by the west, complied with international law.

The Russian leader had reportedly agreed with Putin and Merkel reportedly agreed that more observers from the Organisation for Security and Co-operation in Europe (OSCE) should be deployed in east Ukraine. Existing observers were refused entry to Crimea by pro-Russian checkpoint guards.On Saturday, Russia vetoed a US-drafted motion in the UN security council in New York, which had declared the Crimea referendum invalid. China – a consistent ally of Moscow – abstained.

Ukraine's acting prime minister, Arseniy Yatsenyuk, has promised to take action against separatist "ringleaders" who, he said, had compromised his country's independence "under the cover of Russian troops". He said: "We will find all of them – if it takes one year, two years – and bring them to justice. The ground will burn beneath their feet."The conflict spread from the ground to the internet, with several Nato websites targeted by hackers calling themselves CyberBerkut, after the Ukrainian riot police who were disbanded by the Kiev government. Crimean officials said their referendum website was also hacked.

Pro-unity rallies took place at the Maidan in Kiev on Sunday, the scene of Ukraine's revolution that led to President Viktor Yanukovych abandoning his office and fleeing to Russia last month. Some of those who attended were Crimeans who opposed secession and said they had left the peninsula in recent days amid threats and pressure.

Antonina Danchuk, 30, who lived in Simferopol until two years ago and studied Greek and English at its university, described the referendum as a "fake". "It's illegal," she said. "My Crimean friends who are there are afraid to go out and build their own Maidan. They're not voting. People with Russian passports are being allowed to vote."

A Ukrainian member of the Maidan self-defence forces rides a bicycle next to EU and Ukraine flags during a meeting – organised as reaction to the referendum – at Independence Square, Kiev. Photograph: Robert Ghement/EPA

Danchuk said she was not opposed to Russia , but to Putin and his expansionist policies. "I'm ethnic Russian. But I feel my nationality is Ukrainian. We've stayed in Ukraine for 22 years. We want Putin to leave us alone. We don't want Crimea to be a part of Russia."

Danchuk's mother Larissa, 62, arrived in Kiev on Saturday from Crimea's regional capital, Simferopol, travelling by train. She said she had taken part in anti-secession rallies dressed in the Ukrainian national colours of blue and yellow. She had also taken food to trapped Ukrainian sailors.

"We were protesting outside Simferopol theatre when two cars pulled up. Men with guns got out. They told me: 'If you want to stay alive clear off.' Of course I left. A similar thing happened two days ago at another demonstration next to the [Taras] Shevchenko statue. A man – not local – came up and said: 'What are you doing? Where are your papers?'"

Larissa said she was born in Russia's far east but had lived in Crimea for 37 years. "The whole referendum is taking place at the point of a Kalashnikov. It's improper, and organised by Moscow." She said she did not know how long she would stay out of Crimea but said she wanted to return for her grandson's impending birthday.

Danchuck, her husband Taras and their one-year-old son Lyubomyr had driven to the Maidan in a black saloon car decorated with anti-Putin slogans. One read: "Crimea=Ukraine". Another described the Russian leader as an "executioner". Lyubomyr sat placidly in his pushchair, wearing a yellow and blue scarf, above a sign that read: "Putin is a poo".

Meanwhile, Dave Young, a British expatriate who has lived in Kiev for nine years, turned up at the Maidan on Sunday waving a Union flag with the words: "Ukraine-Great Britain". Young said he was unimpressed by David Cameron's handling of the Ukraine crisis. "His response has been limp and apathetic. He's seemed more concerned with protecting the interests of the City than doing what is right."

Sergei Aksyonov , the pro-Russian prime minister of Crimea, gestures as people celebrate in Lenin Square, in Simferopol on Sunday. Photograph: Max Vetrov/AP

Young said he feared the crisis in Ukraine raised profound questions for Europe and its values. He said: "There is a fundamental argument here about the right of a country to decide its future. God knows how long Russia has been planning this action but it's clear they don't want Ukraine to stand as an independent nation."

"The whole of Europe needs to realise this is a pivotal point. After here, what next? If this state falls where next?"