George Gatteny, of Somerville, said he spotted the suspect among a group of people Sunday at a World War II memorial in the Somerville Veterans Cemetery.
"As they approached the statue, I saw them grab the 3 or 4 mini flags that were placed in [front] of the statue," Gatteny wrote on Facebook. "What I saw him do next pushed me to the point where I said to myself, ’ENOUGH.’ The guy started urinating on the flags."
A woman with the man acted as a lookout while he relieved himself, only stopping when Gatteny yelled at him, he said.
"He stopped urinating, pulled up his pants and started walking out of the memorial," the post continued. "Both of them walked up Broadway laughing. I walked behind them for a while telling everyone I pass what they did."
Gatteny said an elderly man who knew two veterans buried at the cemetery thanked him for taking a stand. But Gatteny was determined to get photographs of the couple so they could be held accountable for their actions, he said.
"What possible reason could they have for desecrating a WW2 Memorial??" Gatteny asked. "There are consequences for our actions. They probably thought no would do anything. They were WRONG!"
Gatteny later met with police to provide photos and statements to investigators.
"They are all over this," according to Gatteny’s post, which generated more than 17,000 comments and was shared more than 39,000 times.
Somerville Police Deputy Chief James Stanford told the Boston Globe that detectives were investigating the incident. No additional information was available as of Tuesday and messages from The Post seeking comment on Wednesday were not immediately returned.
The city’s mayor, meanwhile, slammed the act of vandalism as "despicable," according to a statement to the newspaper.
"If these allegations are true, they are offensive not only to our community and our own veterans, but to our entire nation and those who served and continue to serve," Somerville Mayor Joseph Curtatone said.