Sears Roebuck & Co., the nation's largest retailer, yesterday reported its first increase in monthly sales since April, and several other big chain stores posted substantially greater gains than they have shown in several months.

Sears' sales in October increased 2.7 percent over those for October of last year; Montgomery Ward showed a 9.2 percent gain -- its biggest in a year and a half; and J.C. Penney reported sales climbed 3.4 percent, almost twice its 1.8 percent increase for the year so far.

The apparent improvement in retail sales came just as merchants began the crucial Christmas season, but retailers and analysts were reluctant to proclaim a turnaround in consumer spending.

Forecasting a "competitive" Christmas season, Sears Chairman Edward Telling predicted modest increases in general merchandise sales, but said business is "likely to strengthen as the season progresses."

Since April, Sears' monthly sales figures have been coming up a few percentage points short of the previous year's total. Part of the decrease has been blamed on the state of the economy and part on a decision by Sears to reduce sales promotion and advertising in hopes of improving profits.

A few weeks ago Telling said the retail giant intended to step up its promotional activities, and Sears officials said yesterday the increase in October sales reflected that decision.

"We were stepping up promotions and expected some results like this," said spokesman Ernest Arms. "We're seeing some evidence of strengthening sales, particularly in men's and women's apparel, hardware, microwave ovens, video cassette recorders."

Penney's President Walter J. Neppl cautioned that "continued concern about inflation and the economy in general was reflected in higly selective buying by consumers."

Saying the October sales figures reflected "continued sluggish recovery from the recession," retail specialist Monroe Greenstein of Bear, Stearns & Co. told the Associated press there is another explanation for the relatively strong gains. Because retail sales of a lot of stores were poor last October, "many were coming up against relatively easy numbers," he said.

Improving on last year's sales will be more difficult during the Christmas season, because there are fewer shopping days between Thanksgiving and Christmans this year than last. In 1979 Thanksgiving fell on Nov. 22, but this year it's the 27th; the way merchants figure it, that's five days less business.

The October sales figures released yesterday do not count in the Christmas quarter of most retailers, who end their fiscal year on Jan. 30 -- after the post-Yule clearance sales. Whether the October sales gains will be matched by improved profits won't be known until later this month when third-quarter earnings are reported.

Sears' October sales amounted to $1.50 billion, a rise of 2.7 percent from a year earlier. For the 39-week period to Nov. 1, company sales declined 1.7 percent to $12.71 billion.

K mart said it had a 9.2 percent sales boost for the month -- from $941.9 million last year to $1.03 billion. Sales in the 39-week period rose 12.8 percent to $9.82 billion.

Penney had a 3.4 percent sales rise to $909 million against $879 million in October 1979. Nine-month sales increased 1.8 percent to $7.71 billion from $7.58 billion.

F.W. Woolworth Co. said its October sales came to $540.79 million, a 7.6 percent increase from $502.63 million a year earlier. In the 40 weeks to Nov. 1, sales moved up 6.8 percent to $5.044 billion.

Montgomery Ward, a division of Mobil Corp., said it had a 9.2 percent sales gain for the four weeks ended Nov. 1, from $411.16 million a year ago to $449.18 million this year. The percentage increase was the largest since early last year, the company said. For the 39-week period, sales were up to $3.77 billion.

Federated Department Stores -- which owns Bloomingdales and I. Magnin in the Washington area -- said its sales increased 12.1 percent to $519.8 million from $463.6 million. For the 39 weeks, sales rose 9.2 percent to $4.19 billion; sales of northern California supermarket operations, discontinued in April, were not included.

The Dayton-Hudson chain said it had a 19.4 percent gain in October sales to $331.75 million; for the 39 weeks to Nov. 1, sales rose 17 percent to $2.54 billion.

The 33 percent sales rise for WalMart Stores brought the total for October to $142 million from $107 million a year earlier. For the nine months through Nov. 1 sales rose 31 percent to $1.088 billion.

Tandy Corp., operators of the Radio Shack shops, had a 23 percent sales gain in October to $128 million from $104 million.

October sales gains eleswhere: Carter Hawley Hall, 10.8 percent; City Products Corp., 12.3 percent; May Department Stores, parent of Hechts here, 8.4 percent; Mercantile Stores, 2.4 percent; Jamesway Corp., 18 percent; and Zayre Corp., 21 percent.