A group of 61 U.S. senators, including eight Republican committee chairmen and a number of southern Democrats, defied President Reagan yesterday and introduced a strengthened extension of the 1965 Voting Rights Act, parts of which are to expire in August.

Last month Reagan said he would prefer a straight extension of the the original law, possibly a provision for bilingual voting rights and a "bailout" mechanism to give jurisdictions covered by the act because of their histories of discrimination an easier way to be relieved after a period of good behavior.

Instead, the group led by Edward M. Kennedy (D-Mass.) and Charles McC. Mathias (R-Md.) decided to co-sponsor a bill identical to one passed 389 to 24 by the House in October.

Both the Kennedy-Mathias measure and the House bill extend antidiscrimination laws at the polls indefinitely but provide a bailout provision and extend the requirement for bilingual election materials and voting assistance to 1992.

In addition, they amend the section applying to older voting discrimination cases to provide that mere existence of discrimination constitutes a violation of the law.

Members of Congress say this is necessary to correct recent "misinterpretations" by federal courts which have held it is necessary to prove that officials "intended" to discriminate. This obviously is difficult when those who drew up the election lines or rules are dead.

Reagan has made it clear he would like to stay with the showing of intent.

In announcing that the bill was being introduced in the Senate, Kennedy said the right to vote is "the most basic and fundamental of all our rights and all our liberties. We have come a long way since 1965. But I am concerned about the need for further progress.

"Even now the impressive rise in voter registration is threatened in some states by highly questionable re-registration techniques. Their ostensible purpose is to purge old records, but their timing and methods are clearly discriminatory," he said.

Kennedy said that in addition to the 61, enough other senators have said they will vote for the legislation to break any possible filibuster by opponents.

There are 21 Republican co-sponsors, including Ted Stevens of Alaska, the assistant majority leader; Budget Committee Chairman Pete V. Domenici of New Mexico; Appropriations Committee Chairman Mark O. Hatfield and Commerce Committee Chairman Bob Packwood, both of Oregon, and William V. Roth Jr. of Delaware, chairman of the Government Operations Committee.

There also are a number of southern Democrats, including Dale Bumpers of Arkansas, Jim Sasser of Tennessee, Ernest F. Hollings of South Carolina, Lloyd Bentsen of Texas and J. Bennett Johnston and Russell B. Long of Louisiana.

The issue is scheduled to come before the Senate Judiciary Committee in January.