President Reagan's revised tax-cut plan got a mixed reception from Congress' two tax-writing panels yesterday as House Democrats blasted its proposed cuts for business while senators hinted that they will load the bill with pet amendments.

On the opening day of markup, Democrats on the House Ways and Means Committee branded Reagan's proposed business tax reductions as too generous, charging that they would amount to a subsidy, not a simple writeoff.

At a similar session in the Senate Finance Committee, senators of both parties served notice that they will push to tack on pet proposals, flouting Reagan's plea to avoid another "Christmas-tree" bill.

The Finance Committee also agreed informally to reduce the maximum tax rate on capital gains -- profits from the sale of stocks or other property -- as of yesterday instead of waiting until January, as Reagan would do.

Initial indications from the Ways and Means panel suggested that the markup may take longer than expected, decreasing the likelihood that Congress will complete the tax bill by Aug. 1, as its leaders have promised.

Treasury Secretary Donald T. Regan yesterday charged that Democratic leaders "have wavered and waffled" on the bill, and invited voters and business groups to write their legislators to urge prompt passage.

"Sometimes I think what they're trying to do is to delay in order not to have a tax cut in 1981," Regan told a business group, referring to congressional Democrats.

Meanwhile, debate continued over what kind of tax cut would help the economy more: cutting tax rates by the same percentage for all groups, as Reagan wants, or "targeted" cuts, advocated by House Democrates.

The Treasury published new figures showing that roughly 30 percent of the tax relief under Reagan's plan would go to families with incomes of $50,000 or more, with 61.7 percent going to those in the $30,000-and-above bracket.

Ways and Means Democrats are trying to draft an alternative tax-cut proposal that would "target" the bulk of the tax relief to those in the $10,000-to-$50,000 bracket, a strategy they say they believe is more politically salable.

Regan said yesterday that a more targeted cut "is not what is needed" to spur additional savings and investment, which the administration contends its proposal would do.

The president's proposal calls for three successive tax cuts that together would provide for a 25 percent reduction in tax rates over 30 months. Democrats want to limit any tax bill to two years, saying that any more would fuel inflation.

House Majority Leader James C. Wright Jr. (D-Tex.) yesterday raised the possibility that Democrats might go along with a three-year tax cut provided the final installment were contingent on improved economic conditions. p

However, it was not clear whether his comment was intended as a firm policy statement or was simply his view. A Ways and Means task force working on possible alternatives to Reagan's tax-cut plan has not yet proposed a plan.

The Finance Committee's action on the capital gains tax rate was intended to counter the possibility of an investor freeze on securities and property sales that might result from one of Reagan's tax proposals.

The president has proposed slashing the top tax rate on investment income from 70 to 50 percent, a move that also would reduce the rate on capital gains. But, under Reagan's plan, neither cut would take effect until January, 1982.

Senate Finance Committee member Lloyd M. Bentsen (D-Tex.) expressed fears that a freeze might develop if investors decided to hold their assets until then, and he proposed lowering the capital gains rate as of yesterday.

The maximum tax rate on capital gains is 28 percent. Congress trimmed it from 49 percent in 1979. Both the Bentsen proposal and Reagan's plan would make it 20 percent.

The Democrats' criticism of Reagan's business tax-cut proposal centered on calculations by congressional tax-writing staffs that it would provide $1.07 in tax relief for each $1 invested, at least for new machinery.

Robert S. McIntyre, director of Citizens of Tax Justice, a Ralph Nader organization, charged later that the Reagan plan was a form of welfare that would give many firms what amounts of cash grants that exceed their tax liability.

At the same time, the Democrats pointed to new Treasury figures showing that Reagan's newly "cut-back" business depreciation plan would give firms more tax relief after 1986 than his original, and ostensibly more generous, proposal.

The Treasury calculations show that by 1990 the cost of the newest Reagan business tax-cut plan would increase to $103.1 billion, compared with $92.5 billion under the previous Reagan proposal.

The Finance members' prediliction for tacking on pet amendments could pose a problem for the administration, which wants to keep its tax-cut bill "clean" to encourage speedy passage.

Yesterday, in an effort to nip the amendment fever while he can, Finance Committee Chairman Robert J. Dole (R-Kan.) served notice that the president will not tolerate excessive tinkering with his plan.