Real fear met Halloween revellers parading the streets of downtown Vancouver early Sunday as a group of young adults was sprayed with gunfire.

The violence comes after police put out a public warning they had received information about a high probability of an increase in gang-related activity over the weekend.

But investigators haven't yet made any links between gangs and the two men alleged to have sent four people to hospital with non-life-threatening injuries.

"We put out a notification and then there's a shooting, so the first thought is 'Was it gang related?' We just aren't sure yet," Const. Jana McGuinness said in an interview.

The shooting occurred around 2:15 a.m., in an area of the city packed with clubs and other licensed establishments called "Gastown."

Throngs of costumed partiers were milling the downtown streets, though McGuinness didn't know if the victims were dressed up or out to celebrate.

She said the group, all in their 20s, was standing on a sidewalk near a club when they were shot at and struck in various places including a leg, arm and wrist.

"You've got a chaotic scene," McGuinness said. "You've got witnesses and people out in the street, you've got to try and collect all your witnesses and interview them in a calm setting, and they're obviously traumatized in some cases as well."

Two of the victims were soon released from hospital, including a woman with a chest injury that may not be directly related to the gunfire.

About 30 minutes after the incident, police arrested two men in their 20s not far from the scene. Teams of police were still searching the area for a weapon at midday Sunday.

While paramedics treated the injured, more than a dozen witnesses were rounded up and taken by bus to a police station for interviews.

"Until all the interviews are completed, we're not going to speculate on what precipitated the shooting," McGuinness said. "We're going to move forward with the investigation and see what is revealed."

Whether the "unusual" warning about possible gang-violence will remain in place will be determined after police reassess the situation later Sunday, McGuinness added.