A joint Senate subcommittee ran into opposition yesterday from the Internal Revenue Service and the Securities and Exchange Commission on amendments to the Employe Retirement Income Security Act. Tax and securities officials objected to being left out of proposed changes dealing with administration and extension of benefits in the private pension and welfare law.

Legislation being considered by the finance and human resources subcommittees as three days of hearings opened yesterday is distinct from the reorganization plan announced last week by the White House. That plan, which calls for administrative jurisdiction to be clearly divided between the Labor and Treasury departments, is widely regarded as an interim measure until a better solution is found. Executive branch proposals must be submitted by early 1980.

Meanwhile, the primary bill among the seven ERISA amendments discussed yesterday calls for creating an agency to administer retirement and health benefits. The acting deputy assistant secretary of the Treasury, Daniel Halperin, repeated IRS' objections to the idea.

He warned that reducing the role of the IRS in determining eligibility for tax benefits might "impair equity in the tax system." Halperin said the IRS also opposes some proposed extensions of benefits and pension plan protection because they would aid higher-income-bracket individuals more than those in the lower brackets and would cost the Treasury tax revenues.

Sen. Jacob Javits (R-N.Y.) and Sen. Harrison Williams (D-N.J.) have cosponsored a bill that would allow workers who are covered by an employer plan to supplement their retirement by contributing to an Individual Retirement Account if the employer makes only a small contribution. The additional contributions would be tax deductible.

In other types of tax-favored plans, the worker would be able to contribute the lesser of $1,000 or 10 percent of annual compensation. This is designed to help those who change jobs often. The privilege would not extend to anyone making more than $30,000 a year.

Halperin objected, saying that IRAs already are inherently discriminatory. More than half of the individuals who establish IRAs have incomes of more than $50,000; only 2 percent have incomes below $15,000. Moreover, he said the types of employe tax-deductible plans prposed would cost the Treasury $2 billion annually.

Another important provision of the legislation is intended to aid the surviving spouses of pension plan participants who die before reaching retirement age. Under present law, the spouse of a 45-year-old worker with a fully vested benefit based on 20 years of service would fail to receive any benefit if the worker dies. The bill would provide an annuity or a lump sum for the survivor. The IRS said this proposal raises many difficult questions which it can't answer yet.

The chief objection to the Javits-Williams bill expressed by SEC Chairman Harold Williams centered on the lack of antifraud remedies. In the Daniel case, which is pending before the Supreme Court, a lower court decided that participants in involuntary, noncontributory pension plans are entitled to the SEC's protection. The Senate responded that it never intended pensions to be considered securities and drafted new legislation accordingly. No antifraud remedies are in the amendments to ERISA.

Williams told the hearing yesterday that the SEC's oversight would be eliminated as well for voluntary, contributory plans that invest in the securities of the employer corporation.

Other facets of ERISA's improvement to be considered during the hearings are paperwork reduction, the prudence standard, and the impact of ERISA on state and federal laws.