Bono and David Howell Evans (The Edge) of U2 arrived in Kiev and around 1pm Italian time they improvised a concert in the Khreshchatyk metro station.
The frontman and guitarist of the legendary Irish band sang classics from their repertoire: 'Sunday Bloody Sunday', 'Desire' and 'With or without you'.
The surprise apparition came as airborne warning sirens sounded in the Ukrainian capital.
"The people in Ukraine are not just fighting for your freedom, but for all of us who love freedom," Bono said on a break.
The singer also referred to past conflicts in Ireland and problems with a more powerful neighbor.
"We pray you will enjoy some of that peace soon," Bono said.
At one point, Bono invited a Ukrainian soldier to sing along with him.
The performance took place in front of a small crowd of fans, including members of the Ukrainian Armed Forces dressed in military clothing.