Senate leaders, firing the opening shot in a campaign to push a controversial $15 million security plan for the U.S. Capitol, released figures yesterday showing that threats against members of Congress and the Capitol itself are on the rise, with 114 reported in the first three months this year.

Last year, there were 178 such threats, including 23 bomb threats, and since 1983, when Capitol security was enhanced after a bomb explosion near the Senate chamber, about 150 persons have been arrested on weapons charges, officials said.

The figures were made public as senators Alan K. Simpson (R-Wyo.) and Alan Cranston (D-Calif.) held a news conference to announce details of the security proposal, which includes spending about $13 million to build a wrought iron fence around the Capitol grounds and eight "visitor kiosks," or guard stations, with metal detectors to screen visitors.

The plan, to be introduced soon in the Senate, also calls for a ban on parking in hundreds of coveted spaces on the Capitol's East Plaza and a $2 million expenditure to bring the Capitol police force up to its authorized strength of 1,227 officers.

Simpson, majority whip in the Senate, said it was a plan "to improve security of the nation's Capitol while at the same time protecting the public access to this unique building."

The plan is backed by Senate and House leaders, but it was clear it would face an uphill battle, particularly in the House.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who said earlier this week he would support "whatever the experts on the subject of terrorism tell us we should do," acknowledged yesterday that he was "not that enthusiastic about it."

"I think we have enough security around here. You know, we're the freest nation in the world. We should have security but access."

Rep. Steny H. Hoyer (D-Md.) said he felt a fence "would change the historic character" of the Capitol. "The openness in our democracy is part and parcel of what we are about," said Hoyer, who said he wants to hear more about the necessity of the fence. "One would be foolish to say under no circumstances would I vote for it. But my reaction is it ought to be a last resort."

The comments by O'Neill and Hoyer appeared to reflect the ambivalence felt by some members who remember the Capitol in the days not so long ago when it was completely open, and worry about going overboard with security.

Answering the criticisms about openness yesterday, Cranston, the minority whip in the Senate, said, "This symbol of democracy would be better protected and preserved for the people of this country and as a symbol of freedom for the world by giving it the protection it deserves and by making sure we don't have a catastrophe here . . . if many children, many visitors, many people who work here were killed or maimed in some sort of attack."

Some sources said the most controversial part of the plan is the proposal to ban parking on the East Plaza, where members of Congress, top staffers and the press now park in 850 spaces.