On the eve of the third and final phase of his 25 percent tax cut package taking effect, President Reagan said in remarks prepared for delivery before California Republicans at a fund-raising dinner tonight that Democrats had practiced "pure propaganda" in calling for tax increases and labeling his tax policies as unfair.

Reagan took the offensive, in sharp contrast to recent speeches in which he has seemed defensive about his proposed cuts in the education budget, his military aid to Latin America and questions about the fairness of his economic policies. He was conciliatory in calling for bipartisan support of his Central America policies, however.

The $1,000-a-plate dinner, held in Reagan's honor, was in the Spruce Goose Dome near the famous "Spruce Goose" airplane, the all-wood flying boat built by the late Howard R. Hughes. It is the largest plane in the world and is being restored by Reagan's longtime friend, multimillionaire Jack D. Rather.

In addition to denouncing Democratic proposals for tax increases, the president strongly attacked gun-control legislation as a poor substitute for disarming "thugs." He also said that the communist threat to Central America threatens the American economy because most U.S. foreign trade and oil supplies are transported through the region.

But Reagan began by castigating Democrats for trying to place a ceiling of about $700 on refunds from the 10 percent tax cut scheduled to take effect Friday.

"It took us nearly the entire first year to get our program approved," he said. "And even then, the tax cut had to be watered down. We had proposed 30 percent--they gave us 25 percent and delayed the start of that for several months. The morning after the tax cut passed, the liberals announced the program had failed . . . ."

Reagan then listed his administration's economic accomplishments, including reducing inflation and interest rates and cutting taxes--all of which Democrats contended were not possible under his economic program.

"So much of what they said, and what they are still saying, is pure propaganda," he said. "They even tried to cap the last installment of your tax cut, but despite all their attacks and demagoguery, we stopped them in the Senate . . . . You know, if they had their way, I think our opponents would tax motherhood and apple pie."

Reagan then switched tactics, halting his partisan attack to call for bipartisan support of his Latin American policies. He was conciliatory in referring to opponents of those policies.

He said he appreciated the "sincere motives" of persons pushing for human rights reforms in El Salvador. And he said he agreed that economic assistance, not just military aid, is essential to U.S. efforts in the region.

"But I must come back to the main point," he continued. "U.S. security, the safety of American citizens--that's why Central America matters so much. Either we pay a modest price now . . . or we listen to the do-nothings and risk an explosion of violence that will bring real danger to our borders."

Earlier in the day Reagan traveled to Whittier, Calif., where he addressed a panel of the National Commission on Excellence in Education and met in a school courtyard with a summer school class.