Several police officers have been taken to hospital after being sprayed with an unknown liquid in what is being described as a “deplorable” attack in Derby city centre.

Derbyshire police said a number of officers experienced side-effects from the substance following the incident at about 4.45am on Monday. The force said the officers had since been discharged from hospital and tests were continuing to discover the source of the liquid.

Firefighters were seen examining two plastic drinks bottles containing what the fire service described as an “unexplained and unidentified liquid”.

Another police officer was punched in the face in a separate attack on the same street hours earlier, a police spokeswoman said.

The officer was assaulted as he stepped in to detain a man who had punched two other men outside Living Lounge bar at about 3am. He sustained minor injuries and was taken to hospital before later being discharged, the force said.

Adam Hind, the Derbyshire fire and rescue service station manager, told the Derby Telegraph: “We’re running a series of tests this morning on an unexplained, unidentified liquid. We don’t believe there to be anything hazardous, but we’re just doing a precautionary check.”

Derbyshire police said in a statement: “Following a thorough joint investigation between police and Derbyshire fire and rescue service, the Wardwick has been reopened and we can confirm that no harmful substances were found in the area and that there is no risk to members of the public.”

A Derby University student who witnessed the incident in which several officers were attacked described it as a large fight. “I’ve only been living here for the past four months. I’ve seen fights break out when I’ve been on nights out, but not to this large extent,” said Sam Yeatman, 19, who lives in the city centre.

Derbyshire police’s armed response unit described the incident as “deplorable and totally unacceptable”, and urged witnesses to contact police with any information.