Seventeen veteran antinuclear power activities, protesting the support the nation's financial community has given the atomic energy industry, were arrested here today in the board room of the First National Bank of Boston.

The bank, a leading moneylender to the firm building the embattled $2.6 billion Seabrook plant in New Hampshire, was tapped as the first target in a new campaign dubbed "the Manhattan Project."

The new wave of protest actions planned by the antinuclear Clamshell Alliance is scheduled to climax in New York City with an attempt to shut down Wall Street Oct. 29, the 50th anniversary of the stock market crash.

The Manhattan Project, which will bank on civil disobedience tactics to spread the antinuclear creed, will take "direct action against the economic underpinnings of nukes," according to a statement released by the protesters.

Literally shifting their fight from the streets to the board room, the conservatively dressed demonstrators invaded First National's meeting room shortly after the beginning of business this morning.

Startled bank executives discovered the protesters sitting around the board room table, demanding an end to Seabrook financing, and use of the funds on renewable energy and conservation projects instead.

Outside, police whisked away about 25 picketers chanting, "First National takes the profits, the people take the risks," and brandishing placards reading, "People Before Profits" and "The Nuclear Industry Has a Friend At the First."

Those inside hung a hand-painted banner that read "Stop Seabrook" from the bank's second floor window.

The protesters, who included Clamshell founders Robert Cushing and Guy Chichester, asked for a meeting with First National's directors, but John Calkins, a vice president of the bank, said later, "We did not grant that privilege."

Instead, Boston police were called in on a trespassing complaint. "Unfortunately and regrettably they had to be removed by force," said Calkins of the protesters, who went limp and refused to move.

The demonstrators, many of whom have been arrested in past Seabrook struggles, were dragged out of the bank, placed in waiting police wagons and taken to Boston municipal court.

The protesters, all New Hampshire residents, pleaded not guilty to simple trespass charges and were released on their own recognizance. The cases were continued to Sept. 13 for all but one woman who said she would not return. She was found guilty and given six months' probation.

"We've put First National Bank on notice that they will face a tremendous amount of local pressure in the years to come," said Clamshell member Robin Read. "The financial institutions of this country have to stop supporting nuclear technology."

First National Bank of Boston and Morgan Guaranty Trust Co. of New York lead a consortium of banks behind a $115 million revolving credit account for Public Service Co. of New Hampshire, which owns 50 percent of the Seabrook stock.

The consortium last week extended Public Service's line of credit to July 1980. The debt was to have come due Oct 15, though the deadline has been extended five times in the past several years, according to Public Service spokesman Gordon McKenny.

McKenny rebutted the claims of Clamshell's Roy Morrison that "Public Service Co. is in trouble and it's turning to its friends at the first so it can keep building Seabrook."

The company spokesman pointed to a sale of $50 million in general and refunding second mortgage bonds the last week of September as a sign of corporate good health.