Virginia Gov. Charles S. Robb, faced with a souring revenue outlook, pulled the plug today on $35 million worth of state construction projects, including a new humanities center at George Mason University and long-planned improvements for the Alexandria waterfront.

Robb called his action a "temporary postponement," but did not indicate how long the projects will be deferred. The postponement is the first in a new wave of spending cutbacks the governor is planning to cope with a budget shortfall that has been steadily worsening because of the recession.

Although he refused to specify what additional measures he was considering, Robb emphasized that no item in the state's $6.7 billion budget is off-limits.

"The situation is serious," he declared at a press conference.

The fiscal crisis appeared last summer when Robb ordered a 5 percent across-the-board cut in all state agencies. At the time, budget officials estimated that the shortfall--the difference between projected revenues and actual collections -- would be $64 million during the current fiscal year.

Since then, the state's unemployment rate has jumped, from 7.4 percent to 7.8 percent last month, and sales and income tax collections have plummeted. Some officials have hinted that the shortfall may eventually exceed $100 million, a figure that could have widespread political repercussions if it forces Robb to carve into state aid to local governments.

Robb said today: "There are going to be actions that are going to be regrettably unpopular."

The 33 planned capital projects for which the governor held up state funding could pose a more immediate political problem for the Robb administration by undermining the state's efforts to improve Virginia's traditionally black colleges. Among those affected are eight projects totalling $3.7 million for renovations and construction at two of the black schools, Virginia State University in Petersburg and Norfolk State University.

Only three months ago, Robb had pledged that many of these same projects would be built as part of an ambitious plan designed to demonstrate to the federal government that the state was moving towards desegregating its colleges.

"The state can only finance what the state can finance," said Wayne Anderson, secretary of administration and finance, when asked about the college desegregation plan today. "We recognize the problem . . . . We're not changing the plan. We are having to delay it."

Projects hit in Northern Virginia include George Mason's new $6.3 million Humanities Center, a four-story building that is to house faculty offices, new classrooms and a 360-seat performing arts center. School officials said today they had hoped to begin construction this spring and complete the building by the fall of 1984.

In Alexandria, the governor held up $100,000 in design and planning money for waterfront dredging and pier improvements. City manager Douglas Harman said the cut would be "a serious setback" to the effort to refurbish the city's waterfront.