President Reagan said last night that he opposes extension of a federal program giving up to 14 weeks of extra unemployment benefits to more than 300,000 jobless workers who have exhausted state payments.

With the economy expanding, "We don't believe we should continue with this program indefinitely," Reagan said at his nationally televised news conference. He said job training should be the focus of efforts to help the long-term unemployed.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) had appealed to Reagan earlier in the day to support extension of the benefits.

After the news conference, a spokesman for O'Neill said the speaker believes that the president has "cut the lifeline" for more than 300,000 unemployed workers.

"Without presidential leadership, any measure to help these people would have an extremely hard time. The sad thing is that a lot of these unemployed guys voted for Reagan," said spokesman Christopher Matthews.

The program, which went into effect in September 1982 when unemployment was 10.1 percent, expires the first week in April. All 325,000 current beneficiaries will be removed from the rolls April 7 unless the law is extended. In addition, no new beneficiaries will be accepted.

O'Neill said he would like to extend the program in some form but feared that scheduling a bill would only be an "empty exercise" if Reagan remains "locked in concrete" against it.

In a statement to Congress Feb. 20, Deputy Assistant Secretary of Labor Patrick J. O'Keefe opposed extension of benefits, saying that unemployment is now 3 percentage points lower than when the program started and that extension would cost the Treasury $900 million for six months and $1.85 billion for a year.

The Chamber of Commerce and other business groups also oppose extension. Some argue that extra benefits can retard normal economic shifts by allowing workers displaced by economic changes to delay seeking jobs in other industries or regions.

Labor unions strongly back extension. "The economic recovery, that looks so bright to some, is invisible or nonexistent in countless communities and many states throughout the country," the AFL-CIO said.

Under the unemployment law, jobless workers who meet certain tests can get up to 26 weeks of regular benefits from the state. They can also get up to 13 added weeks -- called "extended benefits" and paid for partly by the federal government and partly by the state -- if they are still unemployed after 26 weeks and the state has high unemployment. Because of changes pushed through Congress in 1981 by Reagan, however, it is harder for states to qualify for this "extended benefits" program than previously. Partly for this reason and partly because economic conditions are improving, only Alaska, Puerto Rico and West Virginia are in the extended benefits program, which had only 21,600 beneficiaries in the week ending March 2.

The program expiring next month is called federal supplemental compensation (FSC) and it is paid for entirely by the federal government. It permits any worker who has exhausted both his initial 26 weeks and his extra 13 weeks to get another 8 to 14 weeks, depending on state conditions. Theoretically, that means an unemployed worker could get up to 53 weeks. However, because only three jurisdictions currently qualify for the 13-week extended benefits program, most of those qualifying are getting only their initial 26 weeks plus the 8-14 weeks of federal supplemental compensation.

For the week ending March 2, according to Labor Department figures released yesterday, about 3.3 million workers were getting regular benefits, 21,600 were getting extended benefits in Alaska, Puerto Rico and West Virginia, and 325,000 were getting the supplemental compensation