US Democrats expressed their condolences and called for action after Saturday's shooting killed at least 20 and injured 26 others in Texas.

The suspect, now in custody, was identified as a 21-year-old white male, named Patrick Crusius from Texas, according to Texas Police Chief Greg Allen.

US President Donald Trump on Twitter called the shooting "an act of cowardice," adding that "there are no reasons or excuses that will ever justify killing innocent people."

"Enough is enough," US House Speaker Nancy Pelosi said in a statement shortly after the shooting happened.

"Today's horrific mass shooting in El Paso breaks the hearts of all Americans," said Pelosi, a Democrat. "Too many families in too many communities have been forced to endure the daily horror of gun violence."

Pelosi slammed Republicans who hold the majority of the Senate for "continued inaction" on passing gun control legislation.

Her remarks were echoed by Democratic presidential contenders, including former Texas congressman Beto O'Rourke, former Vice President Joe Biden and Senator Bernie Sanders.

O'Rourke said in Las Vegas that the El Paso shooting shattered "any illusion that we have that progress is inevitable," referring to tackling gun violence.

"We have to find some reason for optimism and hope or else we consign ourselves to a future where nearly 40,000 people a year will lose their lives to gun violence and I cannot accept that," O'Rourke told reporters.

Biden vowed to take on the National Rifle Association (NRA), the most influential gun-right lobby group in the country, in a bid to contain the rampant gun violence.

"This is a sickness, this is well beyond anything that we should be tolerating," Biden said in a speech to supporters on Saturday. "It's past time we take action and end our gun violence epidemic."

Sanders blasted the Senate for "doing nothing after every tragedy."

"The Senate intimidated by the NRA's power, does nothing," Sanders tweeted. "That must change."

The shooting came just days after two people were shot and killed in a Walmart store in Southaven, Mississippi, south of Memphis. Another three people were killed late July at a shooting incident at the Gilroy Garlic Festival in northern California.

Data from the Gun Violence Archive showed the United States reported 33,015 incidents of gun violence so far in 2019, resulting in 8,721 deaths and 17,286 injuries.