Law enforcement agencies spent less than $40 million of the $50 million appropriated by Congress for security at the Democratic National Convention (search), officials said Friday.

Officials took extraordinary security precautions for the convention, the first national political gathering since the Sept. 11, 2001 terrorist attacks. They closed highways and transit stations near the FleetCenter (search) and cordoned protesters off in a fenced-in demonstration area near the downtown arena.

But only six people were arrested all week — three in a brief scuffle between police and protesters on Thursday, the final day of the convention.

"We expect that the cost will be significantly less than the $50 million," Boston Police Commissioner Kathleen O'Toole said at a news conference flanked by officials from the state police, Secret Service, Boston Fire Department, EMS and other agencies.

The latest estimate for the security costs is between $35 million and $40 million, O'Toole said, although the final amount has not been tallied.

Congress initially earmarked $25 million each for security at the Democratic convention and next month's Republican National Convention (search) in New York. Lawmakers in June added an additional $25 million for each convention.

Police Superintendent Robert Dunford said the unused security money goes back to the federal government. "I would like to spend it all," he said, "but I don't think I can."

O'Toole credited extensive planning and cooperation between the various law enforcement agencies for the lack of convention-related problems.