Thousands marked the anniversary of a bloodily-suppressed student uprising against Greece’s military dictatorship in 1973 on Saturday by joining demonstrations in a heavily-policed city center.

More than 20,000 people turned out for two separate marches -– one organized by students and leftist organizations and the other by the Communist Party (KKE). Some 7,000 police officers remained on standby late into the evening after a day of tight security across the capital, which involved several key metro stations being closed and many central streets cordoned off.

Earlier in the day, representatives of the main political parties visited the National Technical University of Athens, known as the Polytechnic, to pay their respects to the 21 students who were killed in 1973 when army tanks rolled onto the grounds.

The only party leader to visit the university was Alexis Tsipras of the main leftist opposition SYRIZA.

“The message of the Polytechnic will never die,” Tsipras said. “Because it reminds the new generation that the people only have one course toward victory: the course of resistance, of defiance, of rebellion against any kind of tyranny.”

Conservative New Democracy said in a statement that “the message of the Polytechnic is more timely than ever; it is a message of solidarity for our people, for progress and betterment. It is in this direction that all political parties should strive,” it added.

Socialist PASOK called on Greeks to continue their struggle despite the crisis “so we can emerge from the tunnel and make our country stand on its feet again,” while Democratic Left called for “a tireless battle against fascism, racism and violence.”