Alexander K.S. Ho, 34, went to Upper Marlboro yesterday to register to vote just hours before the 9 p.m. deadline because, he said, he was impressed by Walter F. Mondale during Sunday night's presidential debate and wanted to cast his first ballot as a naturalized American for the Democratic presidential candidate.

Ho, who was born in Korea, was joined by Ted Pazulski, 20, a Prince George's Community College student, who also said the debate inspired him to register for the first time. But he registered Republican, he said, because he was impressed by President Reagan.

Ho and Pazulski joined thousands of Marylanders who flocked into the state's elections offices all day yesterday to register to vote in numbers higher than officials could remember.

"We're going crazy," said Robert J. Antonetti Sr., who has been the elections administrator in Prince George's County for 15 years. "This is the busiest we've ever been prior to a presidential election. We picked up close to 5,000 registrants just over the weekend."

State elections chief Willard Morris predicted that by the time the totals are computed, there will probably be at least 2.2 million persons signed up to vote in Maryland -- about 75 percent of those eligible -- the highest total ever for the state.

Elections officials attribute the surge to the emphasis placed on voter registration by both major political parties as well as increased publicity about voting on television and in newspapers.

Long lines were the order of the day and night at registration counters in all jurisdictions. In Montgomery County, elections administrator Douglas Jernigan said that he counted 100 persons at a time waiting to register in his office.

"Oh my gosh," he said during the late afternoon. "We've been flooded." Elections officials continued to accept Montgomery County forms at libraries and shopping malls until 9 p.m., which also was the deadline statewide.

In Anne Arundel County, elections administrator Betty Eby estimated that 1,500 persons had registered in "a steady flow" at four county locations during the day. About 2,000 persons were expected to have registered by day's end.

Howard County officials reported at least 600 registrants yesterday. Before yesterday, more than 76,000 persons had registered this year to vote there, according to Howard County chief election clerk Barbara Feaga, up from 65,801 in 1982.

In most jurisdictions, election officials could not say which party had the registration edge, though in Anne Arundel it was reported that Republicans were registering in slightly greater proportions than Democrats.

Prince George's County's Antonetti said that he expects the county to exceed its goal of registering 300,000 voters. In 1980, he said, about 265,000 persons were registered and eligible to vote in that year's presidential election.

None of the estimates yesterday included the flood of expected mail-in applications.

As long as mail-in registrations are postmarked by last Sunday and received in the elections offices before the end of the day today, Morris said, they will be eligible.

Among those in line yesterday was Jesse Cook, who moved to Upper Marlboro from Philadelphia a year ago and said he had not bothered to change his registration.

"I'm a great procrastinator," he said as he leaned against a wall to complete his form. "It's something that I should have done long ago."

Philip Williams, a Democrat who just bought a home in Laurel and says he will retire soon from his government job, said he decided to switch his District registration to Prince George's after seeing Sunday's debate.

"Sometimes you think it doesn't make a difference, that neither candidate gives a damn," he said yesterday. "My party wasn't coming out and saying what I wanted it to until last night."