Section 98C. Whoever publishes any false written or printed material with intent to maliciously promote hatred of any group of persons in the commonwealth because of race, color or religion shall be guilty of libel and shall be punished by a fine of not more than one thousand dollars or by imprisonment for not more than one year, or both. The defendant may prove in defense that the publication was privileged or was not malicious. Prosecutions under this section shall be instituted only by the attorney general or by the district attorney for the district in which the alleged libel was published.