The Soviet Union is investing twice as much money as the United States on military buildup and is brandishing its might around the world to the point of "undermining America's forward line of defense in Europe and Korea," President Reagan said yesterday.

Reagan, trying to build public support to rebuff any congressional attempts to cut his proposed 14 percent increase in defense spending for fiscal 1984, said the budget he presented to Congress is "a minimal defense budget" and pointed to rising Soviet military power as the "danger we face."

In his regular Saturday afternoon radio address, the president noted that the Soviets showed a willingness to use their growing military power in the "brutal invasion" of Afghanistan.

"Soviet military power has spread around the globe," the president said, "threatening our access to vital resources and our sea lines of communication, undermining our forward line of defense in Europe and Korea and challenging us even at home here in our own hemisphere."

The president spoke from the White House where he stayed this weekend while monitoring activities of four U.S. Airborne Warning and Control System (AWACS) planes sent to Egypt after reports that Libya had planned to aid in an overthrow of neighboring Sudan's government.

In response to the president's speech, Senate Minority Leader Robert C. Byrd (W.Va.), speaking for the Democratic Party, said Reagan's defense budget is excessive, is causing the federal deficit to "skyrocket" and exposes the nation to great economic hardship because of cuts in social spending that Reagan has proposed while increasing the defense budget.

"For example, we do not need two new manned bombers--one of which will be obsolete almost immediately after it is built," Byrd said, referring to the B1B bomber under construction and the "Stealth" aircraft that is scheduled to be built later in the decade.

Reagan has proposed defense spending of $238.6 billion, a 14 percent increase. In the same 1984 budget, Reagan has domestic spending increasing by 2 percent in a year when inflation is expected to rise by 5 percent and unemployment is projected by government officials to remain at about 10 percent. His spending proposals have prompted shrill criticism.

On Wednesday, House Budget Committee Chairman James R. Jones (D-Okla.) said he plans to limit defense spending to a 5 percent increase. Some Republican leaders have said they would cut the defense budget by $20 billion.

Criticism of the defense budget as over-inflated has put Reagan and Defense Secretary Caspar W. Weinberger in a position of staving off attempts to reduce defense spending. In addition to his speech yesterday on the need for added spending on defense, Reagan is scheduled to speak to a convention of the American Legion Tuesday, again on the nation's defense but with an emphasis on foreign policy.

"This the speeches is part of the ongoing process, the offensive to build support for our defense plans," said David R. Gergen, the White House director of communications.

Reagan acknowledged yesterday that the nation's economic problems make this year a "hard time to call for increased defense spending." The president said it was particularly difficult because he has "dedicated his entire political career to reducing government spending."

But the president added, "We've trimmed back our plans for rebuilding defense by more than half. We've hunted for savings in non-essential programs. We've weighed economic risks and economic benefits. The defense budget we finally presented is a minimal budget to protect our country's vital interests and meet our commitments. For those who wish to cut it back further, I have a simple question. Which interests and which commitments are they ready to abandon? "

The president then said the Soviets are engaged in worldwide aggression, including efforts in the Western Hemisphere. He did not elaborate on where the Soviets were exerting a military presence here.