Thousands of people supporting the Gaza-bound aid flotilla took to the streets in several major cities in Europe on Thursday after news of the interception broke.

The protesters decried the Israeli operation and the ongoing offensive in the Gaza Strip.

Some 450 international activists were detained by Israeli naval forces hours earlier in the Mediterranean Sea, Israeli authorities said.

The activists, including European lawmakers, were taking part of a flotilla attempting to break Israel’s blockade of Gaza when their vessels were intercepted, drawing widespread condemnation and sparking protests around the world.

The Global Sumud Flotilla was the largest yet to try to break the blockade, and it comes at a time of growing criticism of Israel's conduct in Gaza, where its offensive has laid waste to wide swaths of territory and killed tens of thousands of people.

Italy’s largest union called for a one-day general strike on Friday.

While the majority of marches were peaceful, clashes erupted between police and pro-Palestinian protesters in Paris and in Barcelona, Spain.

The flotilla, which started out with more than 40 boats, was carrying a symbolic amount of humanitarian aid to Gaza.

Its main goal, they said, remained "to break Israel’s illegal siege and end the ongoing genocide against the Palestinian people."