Simferopol: Thousands rallied across Ukraine in rival demonstrations that turned violent when pro-Russian activists attacked a protest on the Black Sea peninsula, as Russian soldiers tightened their grip across Crimea.

Both Britain and Germany have urged Russian President Vladimir Putin to rein in his soldiers as divisions deepened across the region in the lead-up to Sunday’s referendum on Crimea’s future.

Describing the referendum, which asks residents to vote on Crimea joining Russia, as ''illegal'' German Chancellor Angela Merkel said the vote would violate both Ukraine’s constitution and international law.

Britain’s Prime Minister David Cameron also called Mr Putin on Sunday, urging him to ''de-escalate the situation in Ukraine and to support the formation of a contact group that could lead to direct talks between the governments of Russia and Ukraine''.

His Foreign Secretary, William Hague, told the BBC that if Russian troops moved beyond Crimea into Western Ukraine it would cause ''far-reaching trade, economic and financial consequences'' and ''bring the great danger of a real shooting conflict''.

Mr Putin, however, insists he has the right to protect Russian interests and the rights of ethnic Russians in Crimea.

In the Crimean capital of Simferopol, there were pro-Russian and pro-Ukrainian demonstrations, with protesters on both sides grim-faced and fearful of what may lay ahead for their homeland.

Holding signs portraying Russian leader Vladimir Putin as Hitler, a large rally supporting Ukrainian unity gathered in Shevchenko Park to celebrate the birth 200 years ago of Ukrainian poet Taras Shevchenko.

Joining hands to ''show the world that Ukrainians and Crimeans are one'', the crowd cheered as speaker after speaker condemned the referendum.

''Putin puts his army here and says he wants to protect Russian people,'' said one speaker, Nashida. ''I am an ethnic Russian and I know he does not care at all about me, he only cares about his own political position.''

Protester Yula Yavorska spoke of the fear she felt about the future as she stood with her son in the near-freezing spring air.

''When we are all here together I feel strong, but when I am alone at home I feel so afraid – we are now under Russian occupation, the government of Crimea is against us and I do not know what will happen in the future, for my family or Crimea.''

As a convoy of cars flying the Russian flag drove past the demonstration, the protesters shouted ''Shame, Shame'', but unlike a similar protest 80 kilometres away in Sevastopol, where pro-Russian militants and Cossacks attacked a demonstration with whips, there was no violence.

Across town, pro-Russian demonstrators gathered at the monument to Vladimir Lenin in Simferopol, as reports emerged that Russia was sending in reinforcements to replace the troops already stationed at military barracks and installations around Crimea.

Tensions were high in Yevpatoria, where Russian soldiers had issued an ultimatum to Ukrainian anti-aircraft missile regiment commanders to surrender their weapons by Sunday night local time, their base surrounded by Russian troops.

One protester, Ivan, summed up the feeling of many who were against both the referendum and the presence of Russian soldiers in Crimea: ''I don’t know where this will end – but I know if we go with Russia we’ll go downhill, we will be regressing.''