Foreign Secretary William Hague has sent a message of support for demonstrators who have occupied a central square in the Ukrainian capital, Kiev, telling them he shares their vision of closer ties for their country with the European Union.

Thousands of people have taken part in a series of protests over the past few weeks against President Viktor Yanukovych's decision not to sign an association agreement with the EU, apparently under strong pressure from Russia.

After months of negotiations appeared to have ended in frustration, the EU put any new talks on hold until there is a clear commitment to sign.

Mr Yanukovych is to meet Russian President Vladimir Putin in Moscow tomorrow, and protesters fear that he will sign up to a customs union which would firmly align Ukraine with its giant eastern neighbour instead of with Europe.

Writing in the Daily Telegraph, Mr Hague said: "It is inspiring to see these people standing up for their vision of the future of Ukraine: a free, sovereign, democratic country with much closer ties to the European Union and a positive relationship of mutual respect with Russia. This is a vision I share."

Mr Hague said he was "frustrated" at Mr Yanukovych's decision not to sign the association agreement at a summit last month in the Lithuanian capital, Vilnius.

"Since the end of the Cold War, Britain has been the leading voice supporting the enlargement of the EU to countries of Central and Eastern Europe, and closer ties with other eastern neighbours. We hold to that vision," said the Foreign Secretary.

"Ukraine is an important European country, and with potential to play an influential, positive role both in the region and globally. A stable, prosperous and independent Ukraine is in everyone's interests. I continue to believe that the best way to deliver this better future is through the association agreement."

Mr Hague acknowleged Russia's "strong and legitimate interests in stable relationships with its neighbours". But he added: "Russia cannot dictate terms to them. Russia will also benefit from their economic growth and modernisation and a closer relationship with the EU. This is not a zero-sum game: the choice is not either Russia or the EU. It's about Ukraine finding its true, stable place in the politics of Europe."

He called on the Ukrainian government to launch "rigorous and fair" investigations into allegations of violence committed against protesters and urged the Kiev authorities to engage in "constructive and inclusive discussions to find a peaceful way forward".

"Ukrainians of all political views should work together," he wrote. "Now is the time for leadership, to tackle corruption, and get serious about the rule of law, transparency and accountability. Now is the time to build the real democracy that the people of Ukraine are calling for. We want to see a stable and prosperous Ukraine, secure in its neighbourhood."

However, Ukraine's ambassador to Nato, Ihor Dolhov, insisted that his government had not turned its back on an agreement with the EU. Mr Yanukovych is not planning to join the Russian customs union and the Ukrainian government is continuing to work with the EU on the measures which might ultimately allow it to sign the association agreement, he said.

Mr Dolhov told the BBC Radio 4 Today programme: "The mass rally which started three weeks ago in Ukraine was not a protest rally, it was a support rally for the European integration of Ukraine.

"This is an attempt by people in Ukraine, particularly younger people, who want to be closer to the EU. This is the main foreign policy proriity of President Yanukovych and the Ukrainian government."