OAKLAND, Calif. — This city’s year-old $18 million police radio system failed repeatedly during President Obama’s visit to Oakland Monday and during protests surrounding it.

Many of the 100 police officers assigned to presidential security duty that day were unable at times to communicate through their radios with police dispatchers, and even with each other, during the president’s fund-raiser at a downtown theater, according to the head of the city’s police union.

At one point, officers could not reach dispatchers for about 30 minutes, said Barry Donelan, president of the Oakland Police Officers Association. Another time, some officers reported that the radios failed altogether shortly after the president departed, and some protesters began blocking downtown streets.

‘‘It doesn’t work, that’s the bottom line,’’ Donelan said Thursday.

“Our officers have absolutely no confidence with this current radio system. It puts my officers and the citizens they serve in serious jeopardy because of its unreliability.’’

Sergeant Chris Bolton, chief of staff for Police Chief Howard Jordan, told the San Francisco Chronicle earlier that he was on duty Monday night and was among those who had trouble contacting fellow officers.

‘‘Obviously, we want a reliable radio system,’’ he said.

The radio system that began operating in June 2011 was supposed to be a needed upgrade from the previous analog network reputed to have numerous dead zones in some of the most dangerous parts of town.

But the new digital system has been even more troublesome, plagued by breakdowns and dead spots that have left officers’ vulnerable to blackouts across Oakland and even in many commercial buildings, including the basement of the downtown police headquarters.

Monday’s radio problems were caused by a cooling unit failure on a transmission tower in the Oakland Hills, police and city officials say. It was repaired Tuesday.