Donald Trump has pledged to free people jailed for taking part in the 2021 attack on the US Capitol as one of his first actions if re-elected, calling them "hostages."

The rioters -- egged on by the then-president and fueled by his false claims of voter fraud -- stormed the seat of US democracy on January 6, 2021, in a bid to halt the transfer of power to Joe Biden.

Around 1,358 defendants have been charged in the 38 months since then, according to the latest figures from the Justice Department released last week. About 500 have been sentenced to prison terms.

Trump posted the comments on his Truth Social website, additionally promising to shut down the border with Mexico as part of his first acts if re-elected, without providing details.

"My first acts as your next President will be to Close the Border, DRILL, BABY, DRILL, and Free the January 6 Hostages being wrongfully imprisoned!" he wrote late Monday.

Trump has made previous comments about his hypothetical first day in office, vowing during a televised town hall in December not to act as a dictator except "other than day one."

Nor is it the first time Trump has called his imprisoned supporters "hostages" or suggested they should be freed.

In January, he implored President Biden to "release the J6 hostages, Joe" during a campaign rally in Iowa.

Trump faces four criminal indictments, including two -- one federal and one in Georgia -- stemming from his attempts to overturn the election results after losing to Biden.

Trump is virtually assured of being the Republican presidential nominee now that his last remaining rival, Nikki Haley, dropped out of the race following his overwhelming wins in the primaries.

Trump insists that the support he enjoys among his Republican base has been enhanced, not hurt, by his many legal woes.