Chicago, Illinois - As Chicago welcomes Democrats for their national convention, security services are braced for an array of possible threats – from potentially violent protests to lone wolf bids to avenge the attempt on Donald Trump's life.

America's third-largest city has been hosting political party gatherings dating back to the nomination of Abraham Lincoln in 1860, and law enforcement agencies have determined that its 26th national convention next week will be among the most secure.

More than 50,000 activists are expected to flock to downtown Chicago from Monday for the four-day bonanza, set to be a festive celebration of the Democratic Party's new standard-bearer, Vice President Kamala Harris.

High-profile participants from musicians and Hollywood stars to President Joe Biden are expected, and the Secret Service, FBI, and local police have spent a year planning an ironclad security cordon around the downtown United Center arena.

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Lucas Rothaar, the FBI's top special agent in Chicago, told journalists at a briefing there were no specific concerns but warned of an "elevated threat environment."

"We do acknowledge the vast threats that we face as a nation, running from violent crime to international terrorism, domestic terrorism, and hate crimes, along with a host of other threats," he said.

The July 13 assassination attempt on Trump, which left him with a bloody ear, prompted heavy criticism of the Secret Service over lax security at the Republican's rally venue in Butler, Pennsylvania, where the gunman struck.

Although there is no evidence the shooting was politically motivated, an intelligence assessment by federal and state law enforcement agencies outlines the potential for "retaliatory acts of violence" at the Democratic convention.

The report – obtained by multiple US media outlets, including ABC7 Chicago and CNN – identifies one of the biggest threats as "lone offenders" seeking to advance anti-government beliefs, political grievances, or other ideological positions.

But the risk of more widespread violence has also been accounted for in the convention security plan, which includes detailed procedures for scheduled and pop-up protests.

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Nationwide demonstrations against the Biden administration's role in Israel's assault on Gaza have raised concerns about the potential for a rerun of the 1968 Democratic convention, which saw violent clashes between Chicago police and anti-Vietnam War protesters.

Harris and her running mate Tim Walz have already been grappling with pressure from pro-Palestinian activists interrupting their campaign events in Michigan and San Francisco.

The anti-war Abandon Biden movement is holding its own convention in Chicago on Sunday and Monday, bringing together "leaders and people of conscience from across the nation."

The group pledged "decisive action against the Biden-Harris administration in response to its failure to protect innocent Palestinians" – although it has previewed discussion panels rather than protests.

CODEPINK will also be hosting actions on Sunday, including the floating of a "War Criminals Are Not Welcome Here" message in the Chicago River and a Bodies Outside of Unjust Laws march in the evenign.

Officials have cleared the schedules of dozens of judges and opened up a temporary court to prepare for possible mass arrests, while Democratic leaders have been working with pro-Palestinian groups to head off possible confrontations.

Harris has adopted a harsher tone with Israel than Biden, calling for an immediate ceasefire and pledging to "not be silent" on Palestinian suffering. Still, the vice president affirmed her support for Israel's "right to defend itself" after deadly strikes last month.

Community groups in Chicago believe some protests could see up to 25,000 participants, according to local media.

Tom Carney, a commissioner in Chicago's Department of Transportation, said the city was working to be able to allow protesters "within sight and sound" of the United Center, where the prime-time show unfolds.

Some 2,500 local officers – backed up by hundreds of police from out of town – will be assigned to convention duties.

Chicago Police Superintendent Larry Snelling told journalists that police had been trained to understand the importance of protecting protesters' free speech rights.

But he added: "What we will not tolerate is vandalism to our city. What we will not tolerate is violent activity. If we see that we will put an end to it quickly."