A thief grabbed someone’s iPad in a Metro train Tuesday night and ran out the train’s door. On Capitol Hill earlier the same day, armed robbers took both an iPhone and an iPad.

Police said they made arrests in both cases, which came at a time of heightened concern about robberies in general and about thefts of small, expensive electronic devices in particular.

In the first incident, which occurred about 8 p.m. at the Rhode Island Avenue station on the Red Line, police said the thief was stopped amid commotion at the station by a member of the Guardian Angels, a civilian anticrime group. But someone who apparently was accompanying the thief yanked him — and the iPad — free from the hands of the volunteer.

Police said the thief and his associate then headed for a nearby shopping center, with the theft victim and the Guardian Angel on their heels. Police said they made two arrests.

The earlier Tuesday incident occurred about 1 p.m. after someone got off a Metrobus at Eighth Street and Maryland Avenue NE. Several people who had been on the same bus apparently followed. In a D Street alley, one robber showed a gun and one a knife. They took the iPhone and iPad and fled, police said.

Police said they arrested two men Wednesday and charged them with robbery.

Also Tuesday, police said an iPad was grabbed from someone’s hand in the 700 block of 11th Street NW in the heart of downtown about 7 p.m. Police said they arrested a man nearby.