The Coast Guard says it will ban loitering, stopping, mooring or anchoring boats within 50 yards of 15 Tampa Bay-area bridges during the Republican National Convention.The ban will be enforced 24 hours a day around the three main bridges across Tampa Bay — the Gandy, Howard Frankland and Courtney Campbell — from noon Saturday to 1 a.m. Aug. 31.The ban will be enforced at another dozen bridges in Pinellas County at times when delegate buses are likely to cross them on their way to the Tampa Bay Times Forum."Current analysis indicates that some activist groups are planning maritime activities to make their political views known," the Coast Guard said in the rule, which was published Thursday in the Federal Register. "The geography of the Tampa Bay region makes these 15 bridges a vital component of the regional transportation network."Boats can pass through the security zones but must be "expeditious," the temporary rule says.In a second temporary rule also published Thursday, the Coast Guard said it is establishing moving security zones around cargo ships carrying anhydrous ammonia, liquified propane gas and ammonium nitrate to and from the Port of Tampa. No other boats will be allowed within 500 yards of those cargo vessels from Saturday through Aug. 31.Word of the bans comes as two television news networks were reporting that the FBI and Department of Homeland Security on Wednesday issued a joint warning about potential attacks by anarchist extremists and possible plans to tie up the bay area's bridges during the RNC.The bulletin said that, as of March, the FBI had intelligence indicating individuals from New York "planned to travel to Tampa and attempt to close" all of the Tampa Bay-area bridges during the RNC, according to CNN and Fox News.Transportation is a critical element for the convention, which kicks off Sunday evening with a welcome party for 20,000 journalists, delegates and dignitaries at Tropicana Field in St. Petersburg.Because downtown Tampa doesn't have enough hotel rooms in the immediate proximity of the Tampa Bay Times Forum, delegates, members of the media and other visitors will stay in hotels on both sides of Tampa Bay. Of the approximately 16,000 rooms, nearly a third are in Pinellas County. A fleet of 450 charter buses will carry conventioneers to and from the Times Forum.The FBI-Homeland Security bulletin says that law enforcement agencies believe most protesters at the conventions will obey laws and not commit violent acts, but that anarchists are the most likely exceptions.Fox News reported that the bulletin, titled "Potential For Violent or Criminal Action By Anarchist Extremists During The 2012 National Political Conventions," says extremists probably can't get past the high fences, roadblocks and other tight security that will surround the convention itself.So instead, the network reported, the bulletin said extremists could target nearby infrastructure outside the convention's secure perimeter, including businesses and transit systems and could use tactics that include throwing Molotov cocktails or acid-filled eggs.CNN reported the bulletin notes that anarchists have a history of trying to disrupt major events by blocking streets, intersections and bridges, interfering with business or public transportation and in some instances have "initiated violent confrontations with police." At the 2008 RNC in St. Paul, Minn., the bulletin said, anarchists discussed blocking bridges and skywalks, taking over a radio station, targeting corporations and identifying hotels where delegates were staying."FBI and (Homeland Security) assess with high confidence anarchist extremists will target similar infrastructure in Tampa and Charlotte, with potentially significant impacts on public safety and transportation," according to the law enforcement alert.Richard Danielson can be reached at [email protected], (813) 226-3403 or @Danielson_Times on Twitter.