Hong Kong police laid siege to the Polytechnic University on Monday, firing rubber bullets and tear gas to pin back protesters armed with petrol bombs and other weapons and stop them from fleeing amid fears of a bloody crackdown.

“We’ve seen reports that first responders and other medical staff were detained by law enforcement forces while trying to provide assistance to those who were injured, which is a source of deep concern,” Maja Kocijancic, spokeswoman for foreign affairs at the European Commission, to a news briefing.

“Any violence is of course unacceptable and any action by the law enforcement authorities must remain strictly proportionate and fundamental freedoms, including in particular the right of peaceful assembly and expression, must be upheld.”

The unrest has plunged the Asian financial hub into chaos for almost six months. Demonstrators are angry at what they see as Chinese meddling in Hong Kong’s promised freedoms when the then-British colony returned to Chinese rule in 1997, and say they are responding to excessive use of force by police.

Kocijancic said the European Union’s executive was calling on all sides to exercise restraint and engage in efforts to de-escalate the situation.