In 2001, Bush's limousine was pelted by eggs and stopped for several minutes by a large crowd of protesters. The limousine was forced to turn a sharp corner and speed by the crowd.
Some protesters are complaining about the security regulations, which prohibit signs or placards carried on sticks.
Authorities say they are prepared for anything.
"We pride ourselves on being able to handle large crowds," said U.S. Park Police Chief Dwight Pettiford, whose force is responsible for crowd control for such major events as the Fourth of July fireworks display on the National Mall and last year handled the dedication of the World War II Memorial and President Reagan's funeral. "That's something we've become very proficient at."