US President Donald Trump has held a Nevada campaign rally at an indoor venue despite public health professionals’ warnings against large indoor gatherings during the coronavirus pandemic.

On Sunday local time, Mr Trump, a Republican, railed against his Democratic rival, former vice-president Joe Biden, suggesting he was taking drugs and was soft on crime.

“Biden wants to appease domestic terrorists and my plan is to arrest domestic terrorists,” Mr Trump said to an enthusiastic crowd inside a warehouse in Henderson, near Las Vegas.

“If Biden wins, the mob wins.”

Participants were to have temperatures taken before entry and be given a mask they would be encouraged to wear, the campaign said. However, clearly people in the crowd were seated close together and many did not wear masks.

The Associated Press reported Mr Trump “soaked up raucous cheers” inside the warehouse, packed to capacity with a select group mandated to wear masks if seated directly behind the president.

COVID-19 is killing almost 1000 Americans every day.

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Mr Biden has criticised Mr Trump for holding campaign events that put people at risk of contracting the coronavirus, which has killed more than 194,000 people in the United States and infected more than 6.5 million people since the pandemic began.

Mr Trump played down the virus in its early stages and has alternately embraced and disregarded advice from public health experts, who encourage mask-wearing and maintaining social distance to prevent its spread.

Nevada’s governor Steve Sisolak took to Twitter to denounce the recklessness of the rally, asking why the president would hold a rally for thousands of people while the country was in the middle of a global pandemic.

“Despite reports from his own White House, despite local officials in Southern & Northern Nevada reiterating to the venues the existing restrictions in State emergency directives, tonight, the President is knowingly packing thousands into an indoor venue to hold a political rally,” he wrote.

“The President appears to have forgotten that this country is still in the middle of a global pandemic,” he said.

The majority of supporters chose not to wear masks, except a select group who were seated directly behind Mr Trump. Photo: AAP

The president’s campaign portrayed the rally as an opportunity for supporters to exercise their rights to peaceful assembly under the US Constitution’s First Amendment.

“If you can join tens of thousands of people protesting in the streets, gamble in a casino, or burn down small businesses in riots, you can gather peacefully under the First Amendment to hear from the president of the United States,” spokesman Tim Murtaugh said in a statement.

However, the city of Henderson informed Xtreme Manufacturing on Sunday that the event as planned was in direct violation of the governor’s COVID-19 emergency directives and that penalties would follow, AP reported.

Mr Trump, who is trailing Mr Biden in national polls and in Nevada, has stepped up the frequency of his rallies in recent weeks but has held most of them at outdoor venues or in large open hangars to help minimise risk.

Biden has criticised Trump sharply for his response to the virus and promised a comprehensive strategy to fight the pandemic if he succeeds Trump in the White House.

Mr Trump is in the middle of a western swing that will include a campaign stop in Arizona and a visit to California on Monday to be briefed about the wildfires devastating the west coast.

-with AAP