The robber dumped the wig and was on the verge of getting away from the Charlestown bank he allegedly just held up when his plans were foiled by a Charlestown resident, authorities said Wednesday.

Robert Krol Jr., 32, was ­arraigned Wednesday in Charlestown District Court, where he pleaded not guilty to one count of unarmed robbery. He was ordered held on $50,000 cash bail.

Suffolk District Attorney Daniel F. Conley’s office said Krol entered the Cooperative Bank branch on Main Street around 8:45 a.m. Tuesday. He was wearing a wig, sunglasses, and a baseball cap in an ­attempt to mask his identity.

Once inside the bank, Krol allegedly passed a note to a ­teller giving specific instructions on what he wanted, prosecutors said.

According to prosecutors, the note read: “All 100’s, 50’s, 20’s, no alarm no die [sic] packs be fast.’’

After a teller handed him a small amount of cash, Krol fled the bank, prosecutors said.

But Jim Andrea, 59, of Charlestown, saw the crime unfold while standing in line at the bank. Andrea followed Krol outside and tackled him in the parking lot, prosecutors said.

Andrea, who prosecutors said is about 3 inches shorter and 50 pounds lighter than Krol, held on to the robbery suspect until police arrived.

Andrea was not seriously injured, and Conley praised him for helping law enforcement capture a suspected robber in a neighborhood once notorious for harboring them.

“That’s the real spirit of Charlestown,” Conley said in a statement. “They may not make a movie about him, and we certainly don’t want anyone to get hurt being a hero, but you’ve got to hand it to a guy who can tackle a bank robbery suspect half his age.”

John McCarthy, president of Cooperative Bank, was out of the office Wednesday afternoon and could not be reached for comment.