WASHINGTON (Reuters) - Hundreds of police officers were patrolling around the U.S. Capitol on Saturday, ahead of a rally by supporters of the people who breached the building on Jan. 6 trying to overturn former President Donald Trump's election defeat.

A black eight-foot-high (2.44 m) fence which surrounded the white-domed building for about six months after the attack is back, 100 National Guard troops are on standby and security officials are performing additional checks on travelers arriving at Washington's nearest airport in an effort to prevent violence.

Hours before the rally was set to start, Capitol police officers holding riot helmets and armed with batons and pistols took up positions outside the fence. Municipal buses bringing officers to the site clogged nearby streets. Trucks blocked access points to a staging area between the Capitol building and Union Station.

"On Jan. 6, we knew something was going around but nobody expected what happened. This time, we're expecting the worst," said a police officer on duty near the Capitol, who spoke on condition of anonymity.

Crowds were sparse early in the day. A man named Tim Smith stood shirtless outside the Capitol with a red, white and blue sign with "Loser" painted on it. He said the message was meant for Trump.

Unlike on Jan. 6, when Congress was in session to certify Democrat Joe Biden's election, the Capitol was largely empty on Saturday, with most members out of town.

Organizers of the "Justice for J6" rally said they expected a peaceful event, but U.S. Capitol Police Chief J. Thomas Manger told reporters on Friday there had been threats of violence linked to the rally, where police would work to avoid clashes between Trump supporters and opponents.

More than 600 people have been charged with taking part in the Jan. 6 violence, which followed a speech by Trump at a nearby rally reiterating his false claims that his election loss was the result of widespread fraud. Those claims have been rejected by multiple courts, state election officials and members of Trump's own administration.

Rioters that day battled police, beating them with sticks and metal barricades, smashed their way through windows into the Capitol building and ran through the halls, sending lawmakers and then-Vice President Mike Pence running for safety.

Four people died on Jan. 6, one fatally shot by police and three from medical emergencies. A Capitol Police officer who had been attacked by protesters died the day after and four police officers who took part in the defense of the Capitol later committed suicide.

Almost 50 people have so far pleaded guilty to charges related to the violence, nine admitting to committing felonies. The vast majority of defendants have been released awaiting trial but about 75 are still in custody, according to court documents.

Members of the right-wing groups the Proud Boys, Oath Keepers and Three Percenters are among those charged with storming the building.

"It's ironic that the rallying cries justice for January 6th. I think justice for January 6th would have been the impeachment and removal of Donald Trump," said Representative Adam Kinzinger, one of 10 House Republicans who voted to impeach Trump on a charge of inciting the Jan. 6 violence, in an interview with MSNBC.

Trump was ultimately acquitted by the then-Republican-controlled Senate.

Matt Braynard, a supporter of Trump's false claims that his defeat was the result of widespread fraud who is organizing the rally, said he hoped it would lead to the release of people charged with taking part in the events of Jan. 6.

"What we really want to do is put a spotlight on the mistreatment of these individuals and encourage federal legislators to demand the Department of Justice deliver real justice to these people. And that means, in many cases, dropping charges," Braynard said in an interview on C-SPAN.

While hundreds have been arrested for taking part in the riot -- some of whom posted images online of their activities on Jan. 6 -- questions remain unanswered. No suspects have yet been identified in the investigation into who planted pipe bombs at the Democratic and Republican parties' national headquarters near the Capitol on Jan. 5.

Defense Secretary Lloyd Austin has put 100 National Guard troops on standby to help police on Saturday if needed. Those troops, unarmed except for batons, would be used after local, state and federal law enforcement capabilities had been tapped, a Pentagon spokesman said.

National Guard troops were stationed in and around the Capitol from early January through late May, with as many as 5,200 troops in place at the mission's peak.

(Reporting by Jan Wolfe and David Morgan; Additional reporting by Gabriella Borter, Jim Bourg, Mark Hosenball and Merdie Nzanga; Editing by Scott Malone, Alistair Bell and Daniel Wallis)