Ten-day furloughs affecting more than 3,800 Census Bureau employes here begin next Monday.

Most will lose one day of pay from every salary check for a 20-week period.

Census, which has 5,900 workers nationwide, is also planning to RIF (fire) about 450 employes this year. The ripple effect of the RIFs will mean demotions for another 700 workers.

Officials do not know the precise number of those who will be fired. But at least half of the layoffs and demotions will be in the metro Washington region, mainly at the big Suitland office. Other agencies are also planning furloughs.

Last week the Office of Personnel Management alerted its 5,700 staffers here that layoffs of up to 22 days will begin perhaps as early as April.

Most government agencies (except Defense units) are short of money for salaries. They have been told to make a 16 percent spending cut through Sept. 30.

Congress has still not approved budgets for the fiscal year that began last October, so agencies are operating under a stopgap spending bill that expires at the end of March.

Federal Communications Commission, which has had a partial hiring freeze for more than two years, may have to RIF and furlough workers this fiscal year, and faces a second round of possible layoffs and firings in October.

FCC's current budget has not been approved, nor has Congress given it money to cover the 4.8 percent raise President Reagan granted last October or the executive pay raise cleared this year.

FCC's tentative budget for the upcoming fiscal year called for a cut of 296 jobs. But it appealed those cuts, and the Office of Management and Budget has restored 30 positions--for new high-priority programs.

The way it now looks, subject to congressional approval, FCC will have to eliminate 260 positions beginning in October. Many other agencies are operating partly in the dark, because of congressional failure to tell them how much they can spend for the fiscal year that is already five months gone.