NEW YORK (AP) _ Investor Ivan Boesky has bought a $240 million block of stock in CBS Inc., amounting to 7 percent to 8 percent of the company's shares, the New York Times reported today.

John Guenther, a spokesman for Boesky, declined to comment on the report. A secretary at his company, Boesky Corp., said the financier was not available for comment.

CBS spokeswoman Anne Luzzatto said, ''We are unaware of any unusual accumulation of our stock greater than 5 percent.'' But she added that CBS was determined to remain independent and fight any takeover bid.

Citing ''reliable financial sources,'' the Times said Boesky was expected to file his investment shortly with the Securities and Exchange Commission.

The sources said Boesky accumulated capital for the CBS investment by disposing recently of several holdings in his company's portfolio.

Earlier this week, the American Broadcasting Cos. said it had agreed to be acquired by Capital Cities Communications Inc. There have also been reports that Turner Broadcasting System Inc. was interested acquiring a major television network, possibly CBS.

In recent months, an organization called Fairness in Media has urged conservatives to buy CBS stock to wipe out a perceived liberal bias in the television network's news coverage.

CBS stock opened today on the New York Stock Exchange trading at 1055/8 and jumped 21/2 points to 1081/8 before noon.

Speculation about an attempted takeover led Thomas H. Wyman, chairman and chief executive of CBS, to tell a recent meeting of securities analysts that ''We are in a position to react properly if anything of any real substance should develop.''

J. Kendrick Noble, an analyst who follows CBS for the securities firm Paine Webber Mitchell Hutchins, said he has seen ''no hard information, nothing concrete except for speculation in the media'' to suggest that an investor was ready to make a serious bid to acquire CBS.