Hundreds of health organizations and professionals signed a letter to McDonald's Corp , asking the fast-food chain to stop marketing junk food to children, including with Happy Meals and the Ronald McDonald clown character.

The letter, organized by watchdog group Corporate Accountability International, is slated to run in full-page advertisements in six newspapers -- in Boston, Chicago, New York, San Francisco, Minneapolis and Baltimore -- on Wednesday, a day ahead of McDonald's annual shareholders' meeting.

It cites concerns about the health of U.S. children, ballooning health care costs and an overburdened health care system.

We ask that you heed our concern and retire your marketing promotions for food high in salt, fat, sugar and calories to children, whatever form they take -- from Ronald McDonald to toy giveaways, said the letter.

The letter was signed by more than 550 health professionals and institutions from the American Academy of Child and Adolescent Psychiatry to Marion Nestle, a professor at New York University's Department of Nutrition, Food Studies and Public Health and the author of several books.

McDonald's was not immediately available for comment.

(Reporting by Martinne Geller; Editing by Tim Dobbyn)