California Gov. Edmund G. (Jerry) Brown Jr. flew here tonight to begin his "unofficial" campaign for the Democratic presidential nomination, promising an effort that will "shake things up in the Democratic Party."

Brown was greeted by about 75 supporters, mostly young people, at Logan Airport.Tommorrow, he opens his New Hampshire primary campaign by appearing at two Democratic picnics.

Brown said the news that Sen. Edward M. Kennedy (D-Mass.) may enter the race as a challenger to President Carter "makes it more interesting." He acknowledged that it would put him "in the number three spot," but said that "as an underdog, I have the luxury of being able to say some things others may not be able to say."

The 41-year-old Californian, who won several late primaries from Carter in 1976, said he thought it would be difficult for Carter to be reelected in 1980 if he is the Democratic nominee.

"I believe the Democratic Party deserves a choice," he said, "an alternative for the '80s."

Brown used a helter-skelter airport press conference to rebut charges of political opportunism which his supporters acknowledge will be waiting for him in New Hampshire.

"Opposition to nuclear power and opposition to chronic deficits," he said, "are a responsible philosophy that builds for the future, instead of stealing from it."