The Kremlin hit the country with early morning attacks, targeting cities and bases with air strikes on Thursday, an ambush which the West feared had been coming for weeks.
The president of Ukraine, Volodymyr Zelensky, declared martial law and has called on citizens to volunteer to fight for their country.
Gabriel, 72, who was the lead singer of rock band Genesis, branded the Russian invasion as “one man’s barbaric decision” – referring to president Putin.
On Twitter, he added: “Very shocked to see so many Ukrainians being killed, a totally unnecessary war being deliberately started in Europe.
“This action is a war crime, and whatever else happens, he should never be allowed to leave Russian soil again.”
Gabriel was not the only celebrity to act to the news, with Marvel star Mark Ruffalo, 54, also offering messages of support.
The actor and producer, who plays the Incredible Hulk in films including Avengers: Endgame and Thor: Ragnarok, said on Twitter: “Sending love and good prayers to all innocent people of Ukraine and Russia and Europe who are caught up in this sad and corrosive moment of asymmetrical violence and destruction, especially the young people.
“You have done nothing to deserve this perversion and obscene spectacle.”
Similarly, Top Gear host Paddy McGuinness, 48, said he had to “turn the news off for an hour” to recalibrate after seeing footage from the invasion.
He said on Twitter: “Frightening watching those Russian tanks rolling into the Ukraine, can’t imagine what the people who live there are going through?
“Really hope peace and a diplomatic solution prevails between the two countries.”
Gene Simmons, 72, co-lead singer of the rock band Kiss, wrote on Twitter: “Our hearts and minds are with the brave people of @Ukraine.
“You are not alone!”
Meanwhile, American singer Barbra Streisand revealed that her paternal grandparents emigrated from Ukraine.
On Twitter, the 79-year-old added: “My heart breaks for the courageous people there fighting this Russian invasion.
“Putin’s propaganda about ‘denazification’ as a rationale is one of the great lies of this Century.”
Missy Elliott, 50, who collaborated on a remix of Dua Lipa’s song Levitating with Madonna, shared five love heart emojis, praying for “those who need a peace of mind” on Twitter.
She added: “So many going through things mentally and trying to still navigate through this thing we call Life.
“May peace cover you & fear be removed and You be surrounded with LOVE.”
Actress, singer and dancer Denise Van Outen, 47, also added: “Praying for the people of Ukraine” using a broken heart emoji.
The Kremlin has claimed it is only targeting Ukrainian air bases and other military assets, not populated areas.
Prime Minister Boris Johnson has announced a “severe” package of economic sanctions to pile pressure on Putin and cripple Moscow’s economy.