James M. Mansfield, a manager who has worked for Boston’s Transportation Department for nearly 30 years, was arrested Friday on child pornography charges and put on unpaid leave, officials said.

Boston police arrested Mansfield, 47, of Charlestown, following an investigation alleging that he was “engaging in graphically sexual chat and exchanging pictures of boys in various stages of undress,” police said in a statement Saturday.

Jake Wark, spokesman for the Suffolk County District Attorney’s Office said that Mansfield will be arraigned on Monday at Charlestown District Court.

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Boston Mayor Thomas M. Menino released a statement Saturday saying he was deeply saddened by the allegations.

“As a father and grandfather I am at a loss on what to say about this case other than to assure residents that this employee will no longer be employed by the City of Boston and that we will continue to cooperate in the investigation,” the mayor said. “Without question, this type of behavior is heinous, and many of his colleagues will be shocked by the news and grieving for the victims.”

Menino said the city government is making employee assistance counselors and outreach workers available at City Hall and in Charlestown “where Mr. Mansfield was an involved parent and resident.”

City spokeswoman Dot Joyce said Mansfield was notified in writing Saturday morning that he was placed on unpaid leave pending the outcome of a termination hearing to be held next week.

Joyce said he has worked for the department since 1984. She said his payroll record identifies him as an executive assistant. But he sometimes served as a spokesman for the Transportation Department, and in emails to a Globe reporter late last month he referred to himself as director of community and intergovernmental services.

Police said they found evidence of child pornography on Mansfield’s personal computer while executing a search warrant at his home.

An investigation is ongoing by Boston Police detectives assigned to the Crimes Against Children Unit.

Mansfield did not immediately return phone calls. Joyce said city Transportation Commissioner Thomas Tinlin would not be available for comment.