Rep. Robert A. Young (D-Mo.) yesterday proposed installing hydraulic antiterrorist barriers on the two major roads leading onto the U.S. Capitol grounds as part of an $8.4 million plan to turn the East Plaza into a pedestrian park.

Thirty-inch-high curved steel blades would block vehicle access at the entrances on New Jersey Avenue SE and Delaware Avenue NE. The blades, similar to those used to block access to a U.S. Supreme Court parking lot, would be electronically controlled so they could recess into the ground to allow vehicles to pass.

Young said that "our purpose . . . is to increase the level of security for the Capitol complex without stripping our constituents and the general public of easy access to their own seat of government."

The proposal, drawn up by the staff of Architect of the Capitol George M. White and cleared with congressional leaders, is scheduled to be reviewed in a public hearing today before the House Public Works and Transportation subcommittee on public buildings and grounds, which Young chairs.

White, who oversees the Capitol grounds, said "the beauty of the plan is its simplicity." The Central Plaza, which now serves as a parking lot for 150 House-side and 150 Senate-side cars, "is little more than an incredible indignity" for visitors to Capitol Hill, he said. "Ever since I came to this job 15 years ago, I have been asking myself why do we have to have a supermarket parking lot in the front. Now, with the security concerns, I think we can do away with the parking."

Young complained, for example, that a group of Boy Scouts he was visiting with yesterday had to stand amid parked and moving cars to see the beauty of the Capitol Building.

The plan includes tearing out all the asphalt and replacing it with imported sheets of Belgian granite, a brownish stone. It also calls for new concrete guard booths to replace the current wooden booths, which are not heated or air conditioned, that bar vehicle access.

To fill some of the vacant space on the new plaza, two fountains originally given to the federal government by philanthropist Rose Saul Zalles under a Lady Bird Johnson beautification program in 1968 would be installed.

Security has been particularly tight on the Capitol grounds since a bomb exploded in the Senate wing in late 1983. Every evening since last November, garbage, dump and flat-bed trucks have been placed at the entrance roads to prevent unauthorized vehicle entry, according to Capt. Bob Howe, a spokesman for the U.S. Capitol Police.

Young said he wants the bill passed in the House by September so the Senate would have time to act this year.

Ann Pincus, press secretary to Senate Rules and Administration Committee Chairman Charles McC. Mathias (R-Md.) said that some members of the panel have expressed concern over the cost of the "beautification aspects only. The security system has been endorsed by the committee." She said it is "too early" to tell whether the package could pass the Senate this year.

White said the security devices could be installed in four months, but the granite surface would take longer. He said those parking in the lot would be relocated to other congressional lots.

One of those who would lose his parking place, just a few steps from the building, is White.

"But so what?" he said, pointing out that parking still would still be permitted along the looping drives that surround the Capitol Building. "I can park someplace else."