The Black Sea peninsula, which is under the de facto control of Moscow's troops, went to the polls today in a hastily-arranged referendum on joining Russia.
In London, the demonstrators carried banners, placards and Ukrainian flags and chanted their opposition to Russian President Vladimir Putin's actions.
"Stop Putin's imperialism", "Referendum at gunpoint", "Ukraine: united, peaceful, European" and "Russian army out of Ukraine" read banners carried by protesters.
They chanted "Ukraine united will never be divided", "Putin: hands off Ukraine" and "Russian troops leave Crimea".
Michael Tkachuk, 91, a Ukrainian who came to London after World War II, branded Putin a "bandit".
"The West should do to Putin what they did to Saddam Hussein. All the trouble and problems start in Moscow," he said.
Liza Fimaier, 28, who moved to London three years ago from Russian-speaking south Ukraine, said people were not anti-Russian but anti-Putin.
"I'm half-Russian, I love Russian people and they've always been friends and brothers to Ukrainians and unfortunately just one madman, with his actions, is making us essentially hate him," she told AFP.
