Tensions between police and protesters escalated throughout an anti-fascism demonstration in Worcester on Sunday, leading to seven arrests.

About 40 to 50 protesters marched throughout Worcester Sunday afternoon in a protest billed as an “anti-Presidents’ Day’’ event.

In a Facebook event page for the demonstration, Worcester Antifa — “an autonomous organization fighting against neo-nazis and the far right,’’ according to its Facebook page — wrote of its opposition to President Trump.

“The Trump administration has only been in power for a short time, and already the erosion of civil liberties and denial of our rights has begun. We will not stand for this! Join us in the streets to oppose Trump’s regime!’’ the group wrote on the page.

Police said in a news release that they knew the protest was planned, but that when they tried to find the organizers no one would talk with them.

Many of the protesters were wearing masks, and some carried baseball bats, police said.

The marchers began on Southbridge Street and snaked throughout the city until they reached City Hall. Police said the protesters often walked in the middle of the streets and that officials helped keep vehicles away from the marchers.

Protesters “shouted derogatory comments and abusive words towards the police’’ during their march, police said.

The Worcester Telegram & Gazette wrote that a protester assaulted one of its reporters during the gathering. The paper declined to comment, and police could not be reached to discuss the incident.

After congregating in front of City Hall, the group began walking toward the federal courthouse. Police said they decided to stop the protesters from walking in the middle of the streets, and told them that if they did they would be arrested.

As people continued to walk in the streets, four people were arrested for allegedly disturbing the peace and disorderly conduct. Three others face more serious charges for allegedly resisting or interfering with arrests.

Organizers of the protest could not be reached to comment.

Reena Karasin can be reached at reena.karasin@globe.com. Follow her on Twitter @reenakarasin.