Six people have been injured in a shooting outside a nightclub in downtown St. Catharines, Ont., early Sunday morning, Niagara Regional Police say.

Stephanie Sabourin, a spokesperson for Niagara Regional Police, told CBC News that at approximately 2:30 a.m., officers were called to the area of St. Paul and William streets for shots fired outside the Karma nightclub.

On arrival, EMS located four males and two females suffering from gunshot wounds.

Police say two of the victims are in serious condition and were taken to an out-of-town hospital. The others are being treated at a local hospital.

Moutasem Alhariri was working across the street at Taibah restaurant — which was busy with about 10 customers — when the shooting happened.

Alhariri was in the middle of preparing an order when he heard "four or five loud gunshots" outside. He left the restaurant along with most of the customers to see what had happened.

There was "a lot of confusion" on the street in front of the nightclub, Alhariri said. People were running and scattering, although he said no one was screaming and there weren't any more shots fired.

Everyone who left the restaurant quickly ran back inside, Alhariri said. They were joined by a number of people who were scared by the gunshots and wanted to hide, he said.

St. Catharines Mayor Walter Sendzik called the shooting a "shocking tragedy" that has shaken the city.

While many of us are waking this morning from an amazing Grape and Wine weekend, a shocking tragedy has shaken our city. Early this morning there was a shooting downtown and 6 people were seriously injured. Police are looking for suspects and any tips from the public. —@WSendzik

District Criminal Investigations Branch (CIB) detectives continue to review footage and speak with witnesses, Niagara police said on Sunday. Street closures are in effect in the area.

Investigators are asking anyone with information to call police.