Spectators attending this year's Boston Marathon with backpacks, coolers and other large items will have them subject to search.
Officials on Monday strongly discouraged attendees from bringing such bags and instead urged them to carry belongings in clear plastic bags like those being given to runners to stow their gear.
At least 3,500 police officers will be spread across the eight cities and towns that make up the marathon route. That's more than double the number from last year.
Authorities have been meeting for months to come up with a plan to beef up security following last year's deadly attack. Three people were killed and more than 260 injured after two bombs exploded near the finish line of the race.