By Gram Slattery and James Oliphant

CLINTON, Iowa, Jan 6 (Reuters) - Donald Trump on Saturday downplayed his role in the siege of the U.S. Capitol on the third anniversary of the attack, arguing that those prosecuted for storming the building should be freed.

Speaking at a campaign event in Clinton, Iowa with the first Republican nominating contest little more than a week away, Trump called those jailed in the wake of the January 6, 2021 attack "hostages" and said they had been mistreated by the Biden administration.

"They've suffered enough," Trump said. "I call them hostages. Some people call them prisoners."

Speaking to more than a thousand supporters in a school gymnasium, Trump repeated his unfounded claims that the 2020 election was fraudulent and cast himself as a victim of political persecution.

"I got indicted because I challenged the crooked election," Trump told the crowd.

Trump faces a bevy of state and federal charges for his attempts to subvert the election, but has not been charged with instigating the 2021 insurrection, when a mob of Trump supporters stormed the Capitol as legislators were certifying President Joe Biden's 2020 election victory.

Biden has repeatedly called Trump a threat to democracy on the campaign trail, and that messaging has emerged as an central theme of his campaign so far. Vice President Kamala Harris spoke of the January 6 assault at length during an event in South Carolina on Saturday.

At recent campaign events in Iowa, Trump's supporters -- and even supporters of other Republican presidential hopefuls -- have downplayed the significance of January 6, and many have embraced conspiracy theories regarding the events of that day.

Trump himself has suggested during previous campaign stops that undercover FBI agents played a significant role instigating the attack, an account not supported by official investigations.

More than 1,200 people have been charged with taking part in the riot, and more than 900 have either pleaded guilty or been convicted following a trial.

"It wasn't really an insurrection," said Hale Wilson, a Trump supporter from Des Moines who attended a campaign event in Newton, Iowa earlier in the day. "There were bad actors involved that got the crowd going."

At the Clinton event, Erin George, a local county commissioner, said the prison sentences handed down to the rioters "were 100 percent unwarranted."

Trump was in Iowa to curry support ahead of the state's Republican caucus on January 15, which is the first contest of the Republican presidential nominating contest. He currently leads all competitors by more than 30 percentage points in the state, according to most polls.

Reporting by Gram Slattery in Newton, Iowa and James Oliphant in Clinton, Iowa; Editing by Daniel Wallis and Miral Fahmy