Senate candidates have collected more than $124 million for next week's elections, with the 29 incumbents whose seats are at stake raising 2 1/2 times as much as their challengers.

In a study of 1983-84 Senate campaign financing through Oct. 17, Common Cause, the self-styled citizens lobby, also reported yesterday that the 65 current candidates have spent $109.7 million of the $124.3 million.

Twenty-five Senate candidates, almost all of them incumbents, each got more than half a million dollars in contributions from political action committees (PACs), Common Cause said. By contrast, it added, only 14 Senate candidates raised that much during the 1981-82 election cycle.

Leading the list this year were Rep. Phil Gramm (R-Tex.) and Sen. Charles H. Percy (R-Ill.), with more than $1 million each in PAC contributions. Sens. Thad Cochran (R-Miss.) and Rudy Boschwitz (R-Minn.) were over the $800,000 mark in PAC money.

The three biggest spenders were Sen. Jesse Helms (R-N.C.), $13.9 million; Gov. John D. (Jay) Rockefeller IV (D-W.Va.), $9.3 million; and Helms' opponent in the hard-fought North Carolina race, Gov. James B. Hunt (D), $7.7 million.

The study showed that spending in the first 17 days of October totaled a fast-paced $14.2 million, while fund-raising fell behind, at $11.5 million.

Common Cause said an analysis indicated that Senate campaign spending had almost doubled since 1982, from the standpoint of money spent per eligible voter.

A number of the biggest states -- including California, New York, Florida, Ohio and Pennsylvania -- are not involved in this year's Senate contests. As a result, Common Cause said, the 33 Senate races this year involve a total voting-age population of only 86 million people compared with the 124.7 million affected in 1982.

From that perspective, the study said, Senate campaign spending rose from 66 cents per voting-age individual in 1982 to $1.28 now.

The candidates with the most cash on hand as of Oct. 17 were Sens. J. Bennett Johnston (D-La.), $1.5 million; Bill Bradley (D-N.J.), $1.1 million; Sam Nunn (D-Ga.), $844,000; Howell Heflin (D-Ala.), $709,000, and Hunt, $705,000.

Johnston has won reelection in Louisiana's open primary and, except for Hunt, the others are expected to be easy winners.

Incumbents collected $66.3 million of the $124.3 million raised through Oct. 17, an average of $2.3 million each, the study found. Their rivals raised $25.7 million, an average of $919,000.

The eight candidates for open seats, meanwhile, raised $32.2 million -- an average of more than $4 million.

The National Conservative Political Action Committee announced yesterday that it has filed complaints with the Federal Election Commission against 11 liberal tax-exempt groups for "gross violations" of federal election laws.

Chairman John T. (Terry) Dolan said NCPAC had "amassed overwhelming evidence that these groups have sent out extensive mailings which make partisan points and urge their recipients to vote against conservative candidates, especially the president. Yet it is this very type of activity in which FEC and IRS regulations forbid a nonprofit organization from engaging."

NCPAC named the National Organization for Women, Gray Panthers Project Fund, National Mobilization for Survival, Environmental Task Fund, Committee for Sane Nuclear Policy, Ground Zero, Disarm Education Fund, Wilderness Society, National Women's Political Caucus, Coalition for a New Foreign and Military Policy, and the American Civil Liberties Union.

The ACLU assailed the FEC earlier this week for taking NCPAC's complaint against it seriously, and said it was confident that its criticisms of President Reagan were protected by the First Amendment.