A dramatic increase in bank robberies -- up nearly 50 percent last year in the District and even more in Baltimore -- has prompted police in both cities to institute special security measures to deal with the growing crime problem.

In Baltimore, the robberies have become so frequent that a special force of undercover police "plants" and shotgun-toting officers began appearing at various banks this week. In the District, members of the police department's special operations division also have been planted in plain clothes at some banks and the division is supplementing the department's usual patrols at some "high-risk" financial institutions.

Banks may have become such a popular target because they appear "easier" to rob than stores, according to one Baltimroe police official. Bandits often use a threatening note as their "weapon" rather than a gun and bank employes are instructed to hand over the money to avoid violence.

Whatever the cause, the high increase has been confined mostly to the two urban centers. Neither the Maryland nor Virginia suburbs have reported an increase in robberies between 1979 and 1980. Indeed, some jurisdictions actually reported a drop in the number of bank robberies.

In the District, it was a different story. There were robberies at 88 banks, savings and loans and credit unions in 1980 -- nearly half again as many as in 1979, according to police statistics. Nine of those robberies occurred in the last half of December, and another four have occurred since the new year began, according to Capt. Ronald Crytzer, commander of the robbery branch.

That's a "significant upsurge, so naturally you deploy what forces you have," Crytzer said yesterday. Police turned to the special operations division, and is now using those officers either in undercover positions or on "highly visible" patrols around some of the city's approximately 300 financial institutions where robberies are believed most likely to occur, Cryzter said.

So far this year, police made arrests in three of the four robberies that have occurred, according to Crytzer.

In Baltimore, police have seen what one law enforcement official called a "staggering increase in bank robberies." There, the number of robberies at banks and savings and loans increased by about 68 percent last year, and the city has experienced six more robberies since the new year began.

One of those robberies was interrupted this week by two officers, one toting a shotgun, who had gone on patrol in the city's new program only seven minutes before the robbery attempt, according to a police spokesman. As one officer stood with his shotgun outside a Maryland National Bank branch Wednesday, his partner walked inside and was warned by a teller that a man at another teller's window was robbing the bank. The policeman inside motioned to his partner to come in, then pulled his service revolver and ordered the robber to stop. The suspect was arrested moments later.

Several hours later, though, a bandit pulled off a successful robbery at a bank that the special patrol had visited several times that day.

The Baltimore bank detail, which will employ scores of officers, according to spokesman Dennis Hill, will also do undercover work. Officers will be planted inside banks as customers or employes and others will play decoy roles outside the bank, perhaps as a street sweepers or passersby, Hill said. Canine teams and police helicopters will also do bank surveillance. And police computers have been used to work out schedules for the special patrols so they hit banks at "high-risk" hours calculated by studying previous robberies.

In the majority of bank robberies, a note demanding money and threatening violence is used most often and guns are seldom brandished, officials in both cities said. And according to a national banking security specialist, employes are taught to stay calm and obey the robber's instructions rather than fight back.

Though this may make bank robbery seem enticing, said Keith Marshall of the Bank Administration Institute, the apprehension rate in bank robberies is higher than in any other crime. Nationally, nearly 70 percent of bank robberies in 1979 ultimately ended with arrests, said Marshall. And in the District, Crytzer said that police made arrests in more than 75 percent of the bank robberies last year.