Democratic presidential contenders paraded before Hispanic audiences large and small today in an effort to lay claim to a growing constituency that gave nearly one-third of its votes to President Reagan in 1980 but is becoming increasingly sour toward his administration.

The occasion was the 54th annual convention of the League of United Latin American Citizens (LULAC), the nation's oldest Hispanic civil rights organization, which along with other Hispanic groups is attempting to register more than 1 million new voters before the 1984 elections.

The 100,000-member league does not endorse candidates. But the group is considered to have political significance because most of its members are in Florida, the Southwest and key urban centers, including major cities--Miami, Denver, San Antonio and Santa Fe--with Hispanic mayors.

Conference leaders acknowledged that Hispanics, 30 percent of whom are in the country illegally, have not been the most dependable voting block. But they maintained that appearances by five of the six announced Democratic candidates and the leading advocate of a black candidacy demonstrate growing Hispanic political potential.

The first candidate to appear today, Sen. Alan Cranston (D-Calif.), denounced an immigration revision measure now pending before the House as "unjust, unfair and unworkable," and said some of its provisions reminded him of fascist Italy and Nazi Germany.

Cranston called attention to his support for key Hispanic issues, including voting rights, civil rights and bilingual education. He was applauded loudly when he said the Reagan administration "consistently pursues a domestic policy based on privilege and a foreign policy based on paranoia." He did not mention that he favors normalization of diplomatic relations with Cuba.

(In an interview published earlier this week, Cranston said through an aide that if elected he would extend diplomatic recognition to Fidel Castro's government. "There would be no preconditions," the aide said.

(Asked today about that position, Cranston said that the aide, chief campaign press secretary John Russonello, had stated incorrectly that no preconditions would be involved. The senator said he would seek to improve relations with the Soviet Union before initiating talks on Cuban recognition, and that the process probably would take several years.)

Sen. Ernest F. Hollings (D-S.C.) had to compete for attention with league committee meetings and internal politicking stemming from a vibrant five-way race to succeed departing President Tony Bonilla of Corpus Christi, Tex.

Hollings barely touched on traditional Hispanic concerns. He focused instead on the administration's general economic policies and alleged de-emphasis on human resources programs. He characterized as hypocritical policies that allowed corporations to deduct legal fees but opposed legal services for the indigent.

"Legal services for the poor? We have legal services for the rich," he said. "In fact, if they cut off legal services for the rich Washington would become a ghost town."

One of the most enthusiastic receptions was given to Operation PUSH President Jesse L. Jackson, who called on the delegates tonight to form a black-brown coalition that he said could add 24 to 30 new House members from those groups.

"We can redirect the course of this nation if we do it together," Jackson said, later adding, "We will change the course of American history in 1984."

He also called on his audience to be more demanding of the American business establishment. "We want more than full employment," Jackson said. "We want full development, our share of work and our share of wealth."

Late tonight, as fireworks from the annual Freedom Festival sponsored by Detroit and its Canadian sister city, Windsor, Ontario, boomed over the Detroit River, Sen. John Glenn (D-Ohio) praised the accomplishments of Hispanic servicemen, who he said have received more Medals of Honor--37--than have members of any other American ethnic group.

Glenn criticized the Reagan administration as insensitive to Hispanic concerns. "When the Republican convention nominated Ronald Reagan for president, the doors of opportunity began slamming shut in this country," he said. "And now it's up to us to start forcing those doors back open."

Glenn was preceded in his remarks by Texas Gov. Mark White (D).

Reagan and Vice President Bush were invited here, but neither was able to appear, Bonilla said.

Sen. Pete V. Domenici (R-N.M.) did appear today, telling the audience that both parties could be held responsible for the nation's economic ills and promising the group a role in the economic recovery.

"Any economic growth without Hispanics having an opportunity to grow with it . . . is an America that isn't achieving its mission," Domenici said. "It's less than the great American system has to offer, and it's less than any of us can be proud of."