BOSTON (AFP) - A "no-bags" policy is among tightened security measures being introduced for this year's Boston Marathon, organisers said on Wednesday, as the emotional first race since last year's bombings looms in April.

Two bombs hidden inside backpacks exploded near the finish line of last year's marathon, killing three people and injuring more than 250 others.

Security was tightened at major marathons worldwide as a result. This year, not even runners will be allowed to bring bags to keep personal items, the Boston Athletic Association (BAA) said.

"Bags will not be allowed in certain areas at or near the start in Hopkinton, at or near the finish in Boston, or along the course," the BAA said.

"With an increased field size and for everyone's safety, we will work with public safety officials to ensure that we preserve the exceptional race day experience that makes the Boston Marathon an icon in the world of sport while making race day safe and enjoyable for all," it said.

"Please arrive early for the event and be prepared for security screening.

We greatly appreciate your cooperation, patience, and understanding as we work with public safety officials to enforce these policies."

Other banned items include glass containers, bottles with more than one litre of liquid, vests with pockets and bulky costumes or face coverings.

Clear plastic bags will be provided for runners wishing to check a change of clothes for after the race.

"Only this BAA-provided, clear plastic bag can be used for this purpose. No other bags will be accepted. Any items that you would like to have with you at the conclusion of the race must be placed inside the clear plastic bag the BAA will provide," the statement to participants said.

Another consequence of the attack will be the end of the informal practice of allowing racers who were not registered to run for periods along the 42.195km route.

"Anyone on the course for any distance who has not been assigned, or is not displaying, an officially issued bib number from the BAA is subject to interdiction," the statement said.

"Public safety officials and the BAA strictly prohibit unofficial participation and those in violation will be subject to interdiction."

Military marchers and cyclists had in past years also been allowed onto parts of the course as a form of tribute. That will not be the case now.