"UN, take your responsibility and stop the genocide in Afrin," read one banner, referring to the region in northern Syria where Turkey is waging a campaign against a Syrian Kurd militia.
The protesters who came from across Europe also waved Kurdish flags and pictures of the leader of the outlawed Kurdistan Workers' Party (PKK), chanting "Freedom for Ocalan".
There was a strong police presence at the demonstration, which police said attracted 11,000 people while organisers put the number at between 25,000-30,000.
The march has taken place each year in the city that hosts the European Court of Human Rights and the Council of Europe since Ocalan was detained in 1999.
Ocalan, the figurehead of the PKK's bloody insurgency against the Turkish state, remains behind bars on a prison island off Istanbul.
Ankara launched an offensive in the Afrin area of northern Syria last month against the Syrian Kurdish People's Protection Units (YPG) which it considers a terror group but which is allied with US forces in the fight against the
Islamic State (Isis) group.
"(Turkish President Recep Tayyip) Erdogan is becoming very aggressive towards the Kurds and the situation is getting a lot worse, year after year, month after month," said Newroz, who came from Germany to attend the rally
with his sister and friends.
"Erdogan is a 100 percent dictator, every European should know this," added Okce, who also came from Germany.