At least five people were killed and 18 others injured after a gunman opened fire inside a gay club in the US state of Colorado on Saturday night.

The police said the suspect, named Anderson Lee Aldrich (22 years old), is in custody and receiving treatment for injuries.

The police added that two people with "heroic" traits inside the club managed to overpower the attacker.

"Club Q" in Colorado Springs issued a statement on Facebook saying, "The senseless attack on our community has devastated it," and thanked the patrons who "subdued" the gunman.

The police asked people to be patient while they work on identifying the victims and determining the final casualty count.

Colorado Springs Police said they received an initial emergency call about an active shooter at 11:57 PM local time on Saturday.

The suspect was found inside the club when the police entered. Two firearms were found at the scene, and the suspect is believed to have used a long rifle in the attack.

The police did not immediately indicate the motive behind the shooting and said the number of dead and injured may change as the investigation continues.

A spokesperson for the fire department said the injured were quickly transported to hospitals due to the training they received for such events.

The FBI office in nearby Denver said it was assisting local police in the incident.

Police Chief Adrian Vasquez thanked the two club patrons who intervened to stop the shooter.

He said at a press conference on Sunday: "The evidence and initial interviews indicate that the suspect entered +Club Q+ and immediately started shooting at those inside as he advanced within the club."

He added: "While the suspect was inside the club, at least two heroes inside confronted the suspect and fought with him and were able to prevent the suspect from continuing to kill and harm others. We owe them a great deal of thanks."

A statement posted on the "Club Q" Facebook page thanked "the quick reactions of the heroic patrons who subdued the gunman and ended this hate attack."

At the time, the club was hosting a dance party and was planning to hold an event on Sunday evening to celebrate Transgender Day of Remembrance, to remember those killed because of homophobia and gender identity.

The club’s Facebook page was flooded with comments and condolences from around the world.

One person wrote that the club had been "like a home" to them for many years, and that the news of what happened "completely broke them."

They wrote, "[I have met many great people, met my husband there, so it (the place) holds a special place in my heart. Everyone there was always very welcoming and very kind over the years]."

Another comment added: "I am very broken by this news. +Club Q+ has been the heart of our community for a long time and I am devastated and angry that this happened."

John Suthers, the Mayor of Colorado Springs, described the event as a tragedy.

In 2016, 49 people were killed and more than 50 injured in a shooting at the "Pulse" gay club in Orlando, Florida.

Then US President Barack Obama described the attack as "an act of terrorism and a hate crime."