At least 18 people died across Myanmar on Sunday alone during clashes between the police and anti-coup protesters, according to UN Office of the High Commissioner for Human Rights.

"The military authorities must immediately stop the use of force against civilians and allow the population to express their right to freedom of expression and assembly. The European Union resolutely stands with the brave people of Myanmar, who are defending their democracy. The European Union will take measures in response to these developments shortly," Borrell said in a statement published on the EU External Action Service website.

The EU foreign policy chief added that the violence against citizens manifested the Myanmar armed forces' disregard for international law.

"This weekend, the Myanmar military has increased its brutal repression of peaceful protests across the country, leaving many protestors dead or wounded. In shooting against unarmed citizens, the security forces have shown a blatant disregard for international law, and must be held to account. Violence will not give legitimacy to the illegal over-throwing of the democratically-elected Government," Borrell said.

Tens of thousands of people took to the streets in multiple cities across Myanmar on Sunday as mass demonstrations against the February 1 coup have entered their fourth week. At the same time, tensions between the protesters and the police are rising as law enforcement officers turn to more brutal tactics.

On February 1, Myanmar's military overthrew the government and declared a year-long state of emergency hours before the newly-elected parliament was due to convene. State Counselor Aung San Suu Kyi and President Win Myint, along with other top officials accused of election fraud, have been detained. Several western nations, including the US and the UK, have imposed sanctions on Myanmar's top military officials.