President Reagan has at hand a peerless opportunity to prove that he meant it when he said in a March 14 message to Congress, "The American people believe in human rights and oppose tyranny in whatever form, whether of the left or the right."

His host in Indonesia, President Suharto, is a murderous right-winger. He seized power in a coup in 1965, and killed half a million of his compatriots to consolidate it. He has circumvented political opposition by subsidizing two "rival" parties, ensuring that in four elections he has run unopposed. He is as corrupt as Ferdinand Marcos; as repressive as General Agosto Pinochet of Chile; and if he has not matched the record of Pol Pot, the butcher of Cambodia who murdered 2 million Cambodians, he is, on the small island of Timor, which lies about 1,400 miles from Jakarta, trying to wipe out what remains of its shrinking population.

East Timor offers an ideal chance for Reagan to show that he was not kidding when he said in his 1985 State of the Union message that "freedom is not the sole prerogative of a chosen few, it is the universal right of all God's children."

East Timor, a Portuguese colony for four centuries, expected independence when Portugal withdrew. Instead, in 1975, it was invaded by Suharto, who instituted a reign of terror that continues to this day. The most conservative estimates put the East Timor casualties at 100,000. The 550,000 who survived are subject to intimidation, relocation, crop destruction, arrest, torture and disappearance. About 10,000 have fled.

Monsignor Martinho Da Costa Lopes, erstwhile bishop of East Timor's predominantly Catholic population, asserts that his people face ethnic and cultural genocide. Women patients, he says, are given birth-control injections at health clinics without their consent or knowledge.

Rep. Tony P. Hall (D-Ohio), one of the few voices raised on the issue, calls what is happening "the hidden holocaust." He wrote to the president on the eve of the trip, calling on him to urge Gen. Suharto to permit international organizations some access to the suffering island. No reporters are allowed in; official visitors are subject to total control.

Suharto has shown his colors about the free press by banning the entry of two Australian reporters in the president's entourage who wrote about Suharto's graft, and New York Times reporter Barbara Crossette, who delicately criticized his rule in some previsit dispatches without even mentioning East Timor.

The number of political prisoners cannot be exactly ascertained. After a clamor raised by Amnesty International, which Suharto calls "a communist organization," several thousand were released. Now, according to Hall, detainees have been moved to secret prisons and detention camps, where there is no chance of any international human rights group poking its nose.

The resistance, however, soldiers on. Suharto's government calls the guerrillas "bandits" and sets their number at 500. Their friends say they are about 2,000 strong -- enough to tie down 10,000 of Suharto's crack troops.

Despite their courage and their cause, they are not called "freedom fighters" by the Reagan White House. They have not qualified under the so-called "Reagan doctrine," which makes support of freedom-seeking guerrillas automatic. Instead of lending them a hand, Reagan seeks $37 million in military aid for Suharto, who will use it to put them down.

What have they done wrong? They are being persecuted by a dictator who is not a communist. The rationale of Suharto's slaughter was to foil a communist insurrection. In addition to its great wealth, in oil and other natural resources, Indonesia has control of three "choke points" -- strategic passages that Reagan cited when he was defending Marcos.

The State Department maintains that "a basic change for the better is taking place." With Suharto, State has been seeing light at the end of the tunnel for the last 10 years.

According to a White House spokesman, Reagan is not planning to raise the almost forgotten question of East Timor while he is in Bali -- it is an "internal matter." Perhaps he feels it would spoil the visit -- or divert attention from his prime topic, terrorism.

So much for his promise of even-handedness with dictators and a helping hand for liberty-lovers everywhere. He was just making it up as he went along.