MOSCOW, JULY 2 -- Soviet President Mikhail Gorbachev took the offensive against his hard-line critics today at the opening session of a landmark Communist Party congress, declaring that the Soviet Union faces a "dismal" future if his reform program fails.

Addressing a hall packed with party bureaucrats and senior military officers, Gorbachev acknowledged that the crisis confronting the world's second superpower had deepened during his five years in power. But, while conceding that mistakes had been made by the present leadership, he put most of the blame on "the extremely grim legacy that we inherited."

"This will be a time for blunt speaking," said the Soviet leader, setting the tone for the 10-day congress that could lead to the breakup of the party that has held a virtual monopoly on political power here for 72 years.

"The issue today is this: Either Soviet society will go forward along the path of the profound changes that have been begun, ensuring a worthy future for our great multinational state, or else forces opposed to perestroika {economic and political restructuring} will gain the upper hand. In that case -- let us face the facts squarely -- dismal times would be in store for the country and the people," the 59-year-old president said.

The leadership has been bracing for bitter criticism at the congress, the supreme organ of the 20-million-member party, which is to elect a new ruling Politburo and policy-making Central Committee and adopt a new program. The congress will be an important test of Gorbachev's grip on the party following unprecedented criticism of his policies by hard-liners at last month's conference of the newly formed party branch in the Soviet Russian republic.

A taste of what is in store for Gorbachev and his colleagues came at the start of today's session when a delegate from the far eastern city of Magadan demanded that the current leadership not be allowed to conduct the proceedings. The proposal was rejected, but the delagates did win the right right to examine the individual performance of each member of the party's ruling Politburo.

In scenes that would have been unthinkable at the last party congress 4 1/2 years ago, delegates jeered Premier Nikolai Ryzhkov and ideologist Vadim Medvedev as they gave 20-minute reports of their stewardship. Gorbachev's closest ally on the Politburo, Alexander Yakovlev, fared somewhat better with an impassioned defense of glasnost, or openness, and perestroika that seemed to win him the respect of the gathering.

Declaring that perestroika was taking place "50 years too late," Yakovlev described himself as a "happy man" because he had been able to take part in "a great renovation of a great country and its historic entry into the world of freedom." Yakovlev's reputation as the most liberal member of the leadership has made him a frequent target of conservative anger.

Gorbachev called for "more resolute measures" to dismantle the discredited centrally planned economy and replace it with a market-oriented system. But he also distanced himself from the government's latest economic reform package, which unleashed a storm of protest from conservatives and liberals alike when it was unveiled last month.

"The logic, tactics, priorities and sequence of steps toward a market have not been thought out well enough," Gorbachev declared, describing the emphasis placed on a series of early price hikes as "absurd." A wave of panic buying hit the Soviet Union last month when the proposed price rises were announced, forcing the legislature to postpone debate on the issue until the fall.

The Soviet leader's comments suggested that Ryzhkov is being positioned to take the blame for collapse of the government's second economic reform program in six months. The measures were debated in the Presidential Advisory Council and it was widely assumed they had Gorbachev's support.

Ryzhkov, who has served as premier for five years, sat grim-faced on the podium while Gorbachev accused the government of not doing enough to force industry to abandon old-style economic methods. He later complained that the government's measures had not received support "either from the center or at the local level" and had been subjected to severe press criticism.

Although Gorbachev lashed out at critics on both left and right, he seemed to lay special emphasis on his opponents within the Communist Party's vast bureaucracy. He said that some unnamed "leading cadres" preferred to govern by "old methods" and had refused to accept the need for reform "either politically or psychologically."

Gorbachev said the Stalinist policy of central planning had devastated the Soviet economy, resulting in a series of man-made disasters culminating in the 1986 explosion and fire at the Chernobyl nuclear plant in the Ukraine. He also blamed his predecessors for the "militarization of the economy, which swallowed colossal material and intellectual resources," and the "irreparable human losses due to the war in Afghanistan."

Rejecting criticism that the Kremlin had "lost Eastern Europe," Gorbachev said that his "new thinking" in foreign policy had won the Soviet Union respect around the world. He described the former Communist regimes of Eastern Europe as "a variety of the Stalinist authoritarian and bureaucratic system that we ourselves have abandoned."

"We are being accused of leaving without giving battle. It appears that what this means is that we are being advised to resort to the methods we used in the past," Gorbachev said, referring to the Soviet invasions of Hungary in 1956 and Czechoslovakia in 1968.

At the Russian party conference last week, a senior general, Albert Makashov, derided the "so-called victories of our diplomacy" that led to the Soviet army's being driven out of countries it had liberated from fascism.

Gorbachev and his allies appear to have concluded that they must now respond in forceful terms to the criticism of the hard-liners. But they are holding back from taking the next step of forming an alliance with radicals demanding the transformation of the Communist Party into a Western-style democratic party.

While insisting that party members have a right to express minority viewpoints, Gorbachev called for preservation of the principle of "democratic centralism," which excludes the formation of organized factions in the party. The liberal party group Democratic Platform, which claims support from up to 40 percent of the Communists in the country but has less than 2 percent of the delegates in the hall, say they will leave the party unless their proposals are accepted.

Gorbachev rejected calls by the Democratic Platform for the network of party cells in the armed forces and factories to be disbanded, saying this would violate the right of Soviet citizens to "freedom of assembly." He added, however, that other parties should also have the right to recruit members in government institutions now that the Communist Party has officially given up its constitutional "leading role" in society.

After Gorbachev's 2 1/2-hour speech, Democratic Platform supporters said that the Soviet leader appeared to have taken some steps in their direction, but not enough to satisfy them.

"Gorbachev said some things clearer than he has before. He said our problems all flow from the Stalinist model of the economy. Perhaps at the next congress that Lenin was responsible. My opinion is that the trouble goes back to 1848 when Marx wrote the Communist Manifesto," said eye surgeon Svyataslav Fyodorov, a leading party reformist.

The party general secretary and Soviet president holds the middle ground. He appealed on the eve of the Congress for unity in the ranks and will defend stoutly his "perestroika" reforms if they come under attack. He has also indicated he will try to beat off any challenge to his leadership of the party, at least for now. But he believes the party needs rejuvenation if it is to have a real future. A measure of his success will be how many reformist members are finally elected to the new policy-setting Central Committee. ALEXANDER YAKOVLEV

The 66-year-old Politburo member is one of Gorbachev's few clearcut allies in the party leadership. Yakovlev, a former ambassador to Canada, was originally appointed by Gorbachev to take responsibility for culture and propaganda and has been seen as the driving force behind the cultural thaw. His moves to extend openness in the press have earned him the nickname of "Mr. Glasnost." EDUARD SHEVARDNADZE

Shevardnadze, 62, was appointed foreign minister by Gorbachev in July 1985 and used his native Georgian charm to spearhead Moscow's new international offensive. He played a key role in building the new relationship with the United States and in shaping East-West arms deals. YEGOR LIGACHEV

Politburo member Ligachev, 69, has been the standard-bearer of the orthodox right, criticizing the scale of Gorbachev's reforms and expressing alarm at the disintegration of the communist bloc in Eastern Europe. He appeals to an older generation that opposes change, but his star has been waning since 1988. He revived his challenge to Gorbachev by launching the debate about whether one person should hold both the state presidency and party leadership. But his role as leader of the right-wing has been undermined by Ivan Polozkov. IVAN POLOZKOV

Polozkov, 55, Polozkov represents a generation of younger "neo-conservatives" who believe that the party still holds the key to future political power. He has established himself in the vanguard of the party's conservative right-wing by his election to head the powerful 11-million-member Russian Communist Party. At the RCP's founding congress last month, Polozkov criticized Gorbachev saying there was "no crisis of the Soviet Communist Party but rather a crisis of its leaders." BORIS YELTSIN

The rallying figure of the radicals has become the most popular politician in Russia by attacking the pace of Gorbachev's reforms. A flamboyant Siberian, Yeltsin defied Gorbachev's attempts to kick him onto the political sidelines by securing election as president of the powerful Russian Federation. Since then, the 59-year-old rebel has called for Russia to take control of its economy and loosen its ties with Soviet central authority. GAVRIL POPOV

Popov, 53, was elected mayor of Moscow last April and endeared himself to the city's population by calling for soup kitchens to help soften the move to a market economy. He also introduced a system of passports to protect Muscovites' food supplies when thousands poured into the city in a wave of panic-buying. With Yeltsin, he is a founder of the progressive Inter-regional Group in the Supreme Soviet (Parliament) and has made it clear he expects big changes at the Congress. Last week he said that if the conservatives carried the day at the congress he would turn in his party card. VLADIMIR LYSENKO

Lysenko, 34, is the main representative of Democratic Platform, a radical group said by many to represent up to 40 percent of the party's total membership. But it will account for only about 80 out of the 4,700 delegates to the congress. Democratic Platform wants the party to shed its totalitarian past and become a parliamentary-style party. It says its members will break away from the party if real change is not forthcoming.

SOURCES: The Washington Post, Reuter

THE PARTY CONGRESS is the Communist Party's highest authority. Nearly 4,700 delegates participating in the 28th congress that began yesterday in Moscow will decide whether to revamp the country's most powerful institutions - including Mikhail Gorbachev's post of party general secretary.

THE POLITBURO, a group of about a dozen top party members, is in fact the ruling body of the Soviet Union. Politburo members include Gorbachev, Prime Minister Nikolai Ryzkhov, Foreign Minister Eduard Shevranadze and coservative standard-bearer Yegor Ligachev. The Politburo customarily holds weekly closed-door sessions.

THE CENTRAL COMMITTEE is the party's policy-making body and, theoretically, its top organ between party congresses. The 300-member committee includes local party leaders, high-ranking Moscow government and party officials, military and KGB brass and other national figures and elects the general secretary and the members fo the Politburo.