Donald Trump said Thursday he would reverse US President Joe Biden's order clamping down on illegal border crossings, in his first campaign event since becoming a convicted felon.

The expected Republican Party nominee also threatened to impose tariffs on countries that do not stem the flow of migrants into the United States, as he seeks to make electoral capital on an issue polls show resonates with voters.

Asked by an audience member at an event in the must-win state of Arizona what he would do to shut off the flow of people entering the US via the southern border -- including migrants from China and the Middle East -- Trump said he would use economic measures.

"We have tremendous economic power... if China or some other country is behaving badly, we have things called tariffs that are so severe," he said.

"We are going to be so tough and if a country is not going to behave, we're going to tariff the hell out of that country," he added, without giving details on what size or kind of tariffs he would impose.

Trump was in Phoenix for his first event since being convicted of 34 counts of falsifying business records to cover up hush money paid to a porn star.

A sympathetic audience in this  southwestern border state were given the opportunity after a long speech to ask questions of the former president.

While there were some on the cost of living, most were variations on a theme of immigration, an issue polls show is uppermost in voters minds ahead of the November 5 election.

Biden, seeking to neutralize an issue he is widely thought to be performing badly on, this week issued an executive order barring migrants who enter the US illegally from claiming asylum when numbers surge past 2,500 in a day.

The order also makes it easier to deport people back to Mexico.

The curbs will remain in place until numbers fall back down below 1,500 illegal crossings a day.

Biden came under fire from all sides for the policy shift, which uses the same law that Trump once employed to ban migrants from Muslim countries when he was president.

Republicans, meanwhile, lashed it as too little.

Trump on Thursday said he would reverse the order if he won the election.

"On Day One of my administration, I will be rescinding Crooked Joe's outrageous executive order," he told the cheering crowd, blasting it -- without evidence -- as "pro-invasion, pro-child trafficking... pro-drug dealers."

"And I will terminate every single open borders policy of the Biden administration."

Biden won Arizona in 2020 by a wafer-thin 10,000 votes, and the state is expected to be crucial to both men's chances of winning another four years in the White House.

Trump is expected to hold a rally in the swing state of Nevada on Sunday.