Police alleged a 22-year-old gunman walked into Club Q in Colorado Springs just before midnight on Saturday (Sunday afternoon AEDT) and immediately began firing from a "long rifle".

Colorado Springs Police Department Chief Adrian Vasquez said two firearms were found at the scene and two heroic patrons confronted the alleged gunman and stopped the shooting.

"We owe them a great debt of thanks," the chief said.

The 22-year-old was taken into custody about five minutes after police first received a call, they said.

Lieutenant Pamela Castro of the Colorado Springs Police Department earlier had few details beyond the number of dead and wounded. She said the suspect was injured but didn't know how and that the FBI was on the scene.

Attorney General Merrick Garland has been briefed on the shooting, Justice Department spokesman Anthony Coley said.

The FBI said it was providing assistance to Colorado Springs police, but said the police department was leading the investigation.

The police department planned to hold a news conference on Sunday morning (early Monday AEDT).

The latest incident occurred as anti-gay rhetoric has intensified by extremists. In a statement, Club Q termed the shooting a "hate attack".

"Club Q is devastated by the senseless attack on our community," the club posted on its Facebook page.

It said its prayers were with victims and families.

"We thank the quick reactions of heroic customers that subdued the gunman and ended this hate attack," the club said.

Club Q is a gay and lesbian nightclub that features a "Drag Diva Drag Show" on Saturdays, according to its website.

In addition to the drag show, Club Q's Facebook page said planned entertainment included a "punk and alternative show" preceding a birthday dance party, with a Sunday "all ages brunch".

Colorado Springs is a city of about 480,000 located about 112 kilometres south of Denver and home to the US Air Force Academy, as well as Focus on the Family, a prominent evangelical Christian ministry.

In November 2015, three people were killed and eight wounded at a Planned Parenthood clinic in the city when authorities say a man opened fire because he wanted to wage "war" on the clinic because it performed abortions.

And it occurred in a state that has experienced several notorious mass killings, including at Columbine High School in 1999, a movie theatre in suburban Denver in 2012 and at a Boulder supermarket last year.

Democratic Representative Adam Schiff of California said on Twitter he was "sickened and horrified" by the shooting.

"The LGBTQ+ community is once again the target of the most terrible violence," he said.

"And devastating attacks like these will only become more common if we don't fight back. It must stop."

Representative-elect Eric Sorensen, who is Illinois' first openly gay congressman, tweeted that "we must use loud voices to stand up against hate".

"Our country must turn down the hateful rhetoric aimed at our LGBTQ community," he said.

In June, 31 members of the neo-Nazi group Patriot Front were arrested in Coeur d'Alene, Idaho, and charged with conspiracy to riot at a Pride event. Experts warned that extremist groups could see anti-gay rhetoric as a call to action.

The previous month, a fundamentalist Idaho pastor told his small Boise congregation that gay, lesbian and transgender people should be executed by the government, which lined up with similar sermons from a Texas fundamentalist pastor.