The nation's civilian unemployment rate fell from 6.9 to 6.7 percent in January, which is the lowest rate in nearly six years, the Labor Department reported yesterday.

The number of people without jobs last month broke below the 8 million mark -- to 7.8 million -- for the first time since President Reagan took office, and the number employed increased to a record 109 million. According to the Labor Department's survey of businesses, 565,000 jobs were added on a seasonally adjusted basis, for the third-largest job gain in 30 years.

The unemployment rate last month was the lowest since the 6.3 percent rate for March 1980.

Manufacturers, who have been battered by foreign competition, increased employment for the fourth consecutive month. Between last January and September, manufacturers lost 325,000 jobs. However, since October, they have added 185,000 factory jobs, including 35,000 last month.

The White House said the unemployment report "is the best kind of economic news."

"This is the most solid evidence of all that the economy continues its rebound without letup," White House spokesman Larry Speakes said. "The number of jobs created during the Reagan administration amounts to well over 9 million, and we think it'll go through the roof at 10 million this year."

Economists said that the January employment figures suggest that the economy is showing modest improvement, although for technical reasons the numbers somewhat overstate the economy's strength.

"There is no evidence the economy is improving as rapidly as these employment numbers would suggest," said Lawrence Chimerine, chairman and chief economist for Chase Econometrics. "By the same token, manufacturing was up a little. There is an element of a somewhat stronger economy."

"In general, a decline in unemployment of 0.2 [percentage point] in a single month during a period of sluggish economic growth is suspicious," said Gordon Richards, an economist for the National Association of Manufacturers. "It will not be clear until the next few months whether there has been any significant improvement in labor markets."

Janet L. Norwood, commissioner of the Bureau of Labor Statistics, which collects the unemployment data, said yesterday that part of the improved employment picture was due to seasonal adjustment -- changes made in the data to take into account hiring and firing that generally occurs at the same time each year.

But she also said that, despite the adjustment factor, "January was really a much better month than it usually is, and we shouldn't discount these improvements."

Employment in January usually declines because businesses that take on extra workers during the Christmas shopping season lay them off after the holidays. However, this January's declines were smaller than usual, in part because employers hired fewer people than normal during the Christmas season.

For example, the raw data show that 1.5 million people lost jobs last month. But because fewer people lost jobs than usually is the case in January, the figure was adjusted to show a gain of 565,000 jobs.

"In retail trade, much of the increase probably resulted from the fact that fewer workers were hired during the holiday season and, therefore, fewer were laid off in January," Norwood said. The retail sector rose by 207,000 in January and by 832,000 during 1985 after taking into account seasonal adjustment.

Construction employment also rose strongly last month, due to relatively mild weather and a boost in housing starts, Norwood said.

The Reagan administration has forecast an unemployment rate of 6.7 percent by the end of this year. That forecast is based on a 4 percent rate of economic growth for the year, up sharply from 2.3 percent last year.

Many economists say that the economy will not grow that fast and that, later in the year, the unemployment rate could rise or stagnate at current levels.

Another measure of unemployment that includes members of the armed services stationed in the United States fell from 6.8 to 6.6 percent.

The unemployment rate for men dropped from 5.9 to 5.7 percent last month, and the rate for women declined from 6.2 to 6.1 percent. The rate for teens dropped from 18.8 to 18.4 percent.

For whites, unemployment fell from 5.9 to 5.7 percent. Black unemployment also fell, from 14.9 to 14.4 percent. The black teen-age unemployment rate rose slightly, from 42.3 to 42.4 percent, and the rate for Hispanics declined from 10.4 to 10.1 percent.

Recent revisions show the growing number of Hispanics in the labor force now equals 7 percent of the labor force, the department said.