The number of major crimes reported in the District of Columbia last month was 20 percent higher than in July of last year, and police officials say the upsurge is continuing this month.

Serious crimes reported per day last month exceeded 200 on several days, and on one day reached 252, one of the highest single-day totals ever here. The daily average for crimes reported last month was 170, compared with 150 in July 1979.

Police officials cite high unemployment, increased availability of drugs, the hot weather, inflation and low police morale as some of the reasons for the surge.

An increased availability of herion on the streets is producing "a new batch of heroin addicts" who are committing larcenies, robberies and burglaries to support their habits, said Assistant Chief Maurice Turner, head of field operations.

"Any increase in crime is cause for concern, and the figures for July are particularly so," said Mayor Marion Barry. He said he plans to meet with Police Chief Burtell M. Jefferson and City Administrator Elijah Rogers to analyze the figures and, ". . .if they accurately portray such an alarming increase, we will have to find a way to take crime down."

Barry also said that the upsurge may reflect the increase in drug traffic, which, he said, "the police department is battling under very difficult circumstances, none of which has anything to do with budget reduction."

The number of major crimes, including robberies, rapes, homicides, burglaries, larcenies, aggravated assaults and auto thefts, totaled 5,800 in July, compared with 4,800 in July 1979, according to preliminary figures.

Police officials said yesterday incidents of crime are scattered throughout the District and that officers are being overwhelmed by the numbers.

"They are out there trying," said Deputy Police Chief Charles M. Troublefield of the 4th District.

Officials said a number of individuals committing crimes are previous offenders. However, the officials said, throughout the city there is an unusual number of first-time offenders.

In the 2nd District, for example, 88 persons were arrested for burglaries from Jan 1 to July 31 of this year. Of that number, 43 percent had previous criminal records, according to Deputy Police Chief Lloyd Smith of the 2nd District, which includes Georgetown.

Smith said that of 76 persons arrested during the first seven months of this year on robbery charges, 20 percent had criminal records.

Among the first offenders, officials say, are unemployed juveniles who are out of school this summer and some underemployed persons committing crimes to support themselves in these inflationary times.

Deputy Chief James Kelly of the 7th District said the hot weather creates more opportunities for crimes. Kelly said, "Everybody seems to be out in the street at all hours."

Just last Friday, a Senate aide walking to his Capitol Hill home about 2 a.m., was robbed twice within a block of his home. Five nights earlier, a Northeast Washington man was robbed of his money and his car while stopped at a traffic light on H Street in Northeast. And while walking home, he was robbed again.

Some officials said talk of police layoffs, which eventually were averted, had caused considerable concern among officers who feared for their jobs.

Officials said yesterday that they are instituting various task forces to combat the higher crime. In the 2nd District, a K-9 unit has been added to patrol the office buildings along downtown K Street, and a special 30-man casual clothes unit is working in neighborhoods where dramatic increases have been reported.

Meanwhile, D.C. police yesterday released official statistics for the second quarter of this year showing an overall 11 percent increase over the second quarter of last year. Burglaries, robberies, aggravated assaults, larcenies, and auto thefts were up, while homicides and rapes were down.