KINSHASA (Reuters) - At least four civilians and a police officer were killed in eastern Democratic Republic of Congo on Monday in clashes between security forces and protesters demanding the departure of President Joseph Kabila this year, Human Rights Watch said.

The national electoral commission announced this month that an election to replace Kabila, whose mandate expired last December, cannot take place until April 2019. The delay has raised fears of an escalation of violence in Congo, which is Africa’s biggest copper producer.

Activist groups in the eastern city of Goma had called for a general strike on Monday to protest against the election delay.

Unrest broke out by 5 a.m. (0300 GMT) between police and protesters, who barricaded roads and burned tires, local residents and the police said.

Security forces killed at least four civilians and one police officer also died in the clashes, said Ida Sawyer, central Africa director at New York-based Human Rights Watch.

A spokeswoman for the U.N. mission in Congo said it had documented 37 arrests but could not yet confirm a death toll.

Provincial police commissioner Placide Nyembo said that the police officer was beaten to death by protesters and one demonstrator was killed by a stray bullet.

The police also said in a statement that they broke up a planned demonstration in the northern province of Tshopo by protesters with jerrycans filled with petrol.

Deteriorating security across Congo this year, including a spike in militia violence, has prompted fears the country could slip back to the multi-sided civil wars of the turn of the century, when hundreds of thousands were killed in violence and millions are believed to have perished of hunger and disease.

Kabila says the election delays are due to challenges registering millions of voters. His opponents say he plans to change the constitution to remain in power, as other African leaders have done.