TAMPA, Florida — US authorities are braced for violent protests ahead of the Republican presidential nominating convention that nominally kicks off in Tampa, Florida on Monday.

CNN obtained a copy of an alert from the FBI and the Department of Homeland Security warning that anarchists may be plotting violence at the event formally crowning Mitt Romney as President Barack Obama's challenger.

Thousands of demonstrators are expected to converge on this Gulf coast city -- including a large contingent from the Occupy movement that bunkered down for months last year in scores of parks and other sites across the country.

Due to Tropical Storm Isaac, the convention will nominally open Monday and then immediately adjourn to reconvene on Tuesday, when the weather is expected to clear up.

Romney, a former Massachusetts governor and multi-millionaire businessman, will accept the Republican presidential nomination at the end of the week in a raucous finale to several days of political theater attended by tens of thousands of delegates, journalists and party insiders.

CNN said that the FBI-DHS memo warned that anarchists "from New York" were planning to go to Tampa to disrupt the event and might even attempt to shut down the city's bridges using improvised explosive devices.

The advisory warned that a similar plot could be under way for the Democratic National Convention, which is to be held in Charlotte, North Carolina from September 4 to September 7.

"FBI and DHS assess with high confidence anarchist extremists will target similar infrastructure in Tampa and Charlotte, with potentially significant impacts on public safety and transportation," it said, according to CNN.

The memo urged state and local law enforcement agencies to be on the lookout for suspicious people trying to purchase explosive materials or obtain weapons training.

Authorities have said it hard to predict exactly how many demonstrators will swarm this Florida Gulf coast city, but say the numbers could exceed 15,000.

For the most part, authorities are anticipating peaceful protests, but are girding for the likelihood that at least some of the demonstrators will be intent on fomenting trouble.

Tampa's police department spent more than half a million dollars on special khaki uniforms for the roughly 4,000 officers who will police the event. The city also invested nearly $300,000 in the purchase of a truck specially outfitted for police commando units.

Protesters have erected what they called a "Romneyville" near the site of the convention -- an updated variation on the idea of the "Hooverville" tent cities of the Great Depression -- where protesters can be housed and fed.

There is a long tradition of public protest at the quadrennial political conventions, when the two major US political parties choose the standard bearers who will lead them into November's presidential election.

In 2000, the Democratic convention in Los Angeles saw rock-throwing protesters clash with police firing rubber bullets and lobbing tear gas canisters.

Four years later, the Republic National Convention saw parts of New York City transformed into a fortress, and police place some 1,800 people under arrest.

Democrats that same year held their party confab in Boston, Massachusetts, which also took on the aura of a garrison town complete with razor-wire fencing to keep protesters away from the main action.

Protesters in Minneapolis and Saint Paul, Minnesota, host of the 2008 Republican convention, faced off against baton-wielding police in full riot gear, with hundreds of people eventually placed under arrest.

[Image courtesy of PBS Newshour, Creative Commons licensed]