Flanked by posters of alleged members of the Venezuelan gang Tren de Aragua, Trump also said that if elected he would launch a national "Operation Aurora" to target the gang members.
Trump, the Republican presidential candidate, has noticeably hardened his anti-immigration rhetoric in the final weeks of the November 5 election campaign, in which he aims to defeat Democratic candidate Kamala Harris. Illegal immigration is a top voter concern, and Trump is seen by most voters as the person best able to address it, opinion polls show.
"I'm hereby calling for the death penalty for any migrant that kills an American citizen or a law enforcement officer," Trump said to loud cheers from a large crowd of supporters.
Trump has already proposed an expansion of the death penalty for other offenders, including people convicted of sex trafficking women and children.
Nearly half of US states ban the death penalty. While there is a federal death penalty, it is rarely used, according to the Death Penalty Information Center, a non-profit group. An expansion of eligible crimes would require an act of the US Congress.
One of the hallmarks of Trump's third presidential run has been his focus on what he calls "migrant crime", even though academic studies show immigrants do not commit crimes at a higher rate than native-born Americans.
The Harris campaign did not immediately respond to a request for comment related to the death penalty proposal.
Vice President Harris toughened her stance on border security after becoming the Democratic nominee in August, and faults Trump for helping stifle a bipartisan border security bill in Congress earlier this year.
SPOTLIGHT ON AURORA
During a September 10 presidential debate with Harris, Trump claimed that members of Tren de Aragua controlled several dilapidated Aurora apartment complexes - allegations denied by top city officials.
At the rally on Friday, he reiterated a pledge to focus enforcement on Aurora if reelected.
"I will rescue Aurora and every town that has been invaded and conquered," Trump said. "We will put these vicious and bloodthirsty criminals in jail or kick them out of our country.”
Despite Trump's dramatic statements aimed at rousing his base, no town in America has been taken over by migrants.
Aurora Mayor Mike Coffman, a Republican, said in a statement ahead of the rally that “concerns about Venezuelan gang activity have been grossly exaggerated” and welcomed Trump to tour the city.
Major crimes in Aurora have dropped year-on-year, according to Aurora Police Department statistics.
V Reeves, a community organiser with the Housekeys Action Network, said residents were nervous about the Trump visit.
“They're worried about Trump supporters showing up and threatening them,” Reeves said.
At two of the apartment buildings at the centre of the controversy, Reuters observed broken windows and litter on Thursday. One building had portable police camera stations that residents said were installed this week.
Jesus, a 30-year-old Venezuelan who works in landscaping and lives in one of the buildings, said all Venezuelans were being blamed for the actions of a few.
"We are not all bad people," said Jesus, who declined to provide his last name.
The dispute over gang activity in Aurora stems from an effort by the city to force the owner of several apartment complexes where migrants lived to address repairs, trash, pest infestations and other complaints.
A public relations firm hired by the landlord, CBZ Management, said in early August that Tren de Aragua gang members had taken over the properties, according to documents obtained by Reuters through a Colorado Open Records Act request.
CBZ Management did not respond to requests for comment.
The allegations gained national attention in late August when a clip of gun-toting men in one of the apartment buildings went viral.
Cindy Romero, a former resident at the apartment complex, captured the footage with her two security cameras. Speaking at the rally on Friday, she said she was a lifelong Democrat but that she would vote for Trump because of her experience with crime.
