Secretary of State Alexander M. Haig Jr., in a major speech here aimed at countering critics of Western political values and defense plans, said today that "we now have evidence" that outlawed chemical weapons are being used in Southeast Asia.

As Haig spoke, a crowd estimated by police at 30,000 marched here to protest the Reagan administration's defense policies. Some clashed violently with police in what authorities called the city's largest demonstration against a visiting American.

Although Haig did not link the Soviet Union specifically with this potentially serious charge, the context of his speech and a later press conference left the implication that Moscow and its allies had a hand in developing and using these poisonous chemical toxins as weapons. Neither Haig nor his aides did much to dispel that implication.

Asked at the press conference whether these weapons were, in fact, being supplied by Moscow, Haig refused to go beyond his prepared statement. He said more details would be disclosed in Washington Monday and that he included the issue in his speech "to underline the dangers of the double standard" he said was being applied to Western defense efforts.

The Soviet news agency Tass described Haig's charges as monstrous and slanderous, Reuter reported from Moscow. Haig's suggestion of chemical weapons use was "unfounded and false," Tass reported. "In the entire history of international relations no state has used chemical weapons as widely as the United States."

Haig's speech here was meant to be an American-led counterattack against a small but growing and increasingly vocal group of critics--mostly left-wing and youth groups--who are challenging the Reagan administration's defense and foreign policies and are causing concern on both sides of the Atlantic about growing Western European neutralism, pacificism or anti-Americanism.

Thousands of police kept demonstrators out of sight of Haig during his brief visit here. But the protesters in the streets included youth groups from both parties of the ruling political coalition here, a fact that officials said undoubtedly caused considerable embarrassment to the Bonn government.

About 1,000 militants clashed with police on side streets bordering a plaza near the city hall. Police repelled a large group that attempted to crash the barriers that had sealed the city hall area, where Haig went to sign the guest book. Some rioters had axes. A food store was raided. There was some arson, and cars were stoned and some set on fire. Forty-four police officers were injured and about 100 demonstrators were arrested, according to police.

Officials said it was the largest demonstration in memory involving a high-ranking American official. It contrasted sharply with Berlin's traditionally warm welcomes for American officials, which extended even to then-president Jimmy Carter in 1978, when his popularity was not high here.

Aides to Haig suggested privately that it was the commotion in Europe over U.S. neutron weapons and cruise missiles, which the administration sees as a way to deter war, that prompted Haig to expand on this theme and add the material about chemical warfare.

While the United States is being accused by some European critics of dragging its feet on arms control, Haig said, "Others appear to be violating one of the oldest arms-control agreements--that prohibiting the use of toxins."

Haig said that for some time "the international community has been alarmed by continuing reports that the Soviet Union and its allies have been using lethal chemical weapons in Laos, Kampuchea [Cambodia] and Afghanistan." The United Nations set up a group to investigate this, he said, but "reports of this unlawful and inhumane activity have continued."

"Moreover," he claimed, "we now have physical evidence from Southeast Asia, which has been analyzed and found to contain abnormally high levels of three potent mycotoxins--poisonous substances not indigenous to the region. The use in war of such toxins is prohibited by the 1925 Geneva Protocol, and . . . their very manufacture for such purposes is strictly forbidden by the 1975 Biological Weapons Convention."

Haig said the United States is "taking steps to ensure that this evidence is called to the attention of states and that it is provided to both the secretary general of the United Nations" and to the experts investigating such problems under U.N. auspices.

Although Haig and his aides declined to provide any further specifics, apparently in an attempt to avoid upstaging the remainder of a speech they viewed as important, other aides said privately that Haig was referring to a chemical agent called trichothecene toxin, which can cause severe burning, convulsions, massive internal bleeding and death in humans. Time magazine reported this week that it had been gathered from samples taken by Thai patrols in areas of Cambodia where Vietnamese troops have been operating against Cambodian insurgents.

In his speech, Haig said the "democratic revolution" held the best hope for human progress, in comparison to revolutions that lead to repression of the individual. "What have we in the West to apologize for?" he asked.

The danger, he said, that must be confronted was a "double standard" he detected growing in the West "toward appropriate norms of international behavior. One is a supercritical standard applied to those who cherish diversity, tolerate dissent and seek peaceful change. Another is a more tolerant standard applied to those who abhor diversity, suppress dissent and promote violent change."

Haig then noted that the Soviets have occupied Afghanistan since 1979, destroyed much of its culture and exiled one-fifth of the population.

"But why are the voices of conscience among us which cry out against this aggression so muted?" he asked.

"Vietnam, which inspired such widespread concern in the West not long ago, has enslaved its southern populations, has seized Kampuchea Cambodia and now threatens the peace of Southeast Asia. Libya, a country which finances terror and assassinations in countries far from its borders," invades and occupies neighboring Chad and calls it "unification," Haig said. "Where are the demonstrations against these outrages?"

Hopes for eventual mutual arms reductions, he said, "will be doomed if our people succumb to a double standard that falsely blames the troubled state of the world not on aggression but on the effort to defend against it."

In his speech, Haig acknowledged that while debate is not unhealthy, the future of the transatlantic relationship is "being hotly debated" and "both the substance and tone of our debates of late have begun to take a disturbing turn."

Too many people, Haig said, are "prophesying a future devoid of hope."