The shah of Iran left tody en route to the United States with a new arms shopping list potentially worth billions of dollars, while dissidents at home escalated efforts to liberalize his one-man rule.

On the eve of his U.S. visit, a group of prominent Iranians issued a letter to the foreign press calling for the disbanding of the feared Iranian secret police, SAVAK, free elections and other changes that, if enacted, would effectively dismantle the shah's regime.

The letter apparently is aimed at reinforcing critics of the shah in the U.S. Congress who charge his government with massive violations of human rights and argue for a cutoff of arms sales. These congressional critics have teamed with others who argue that sales have already been so massive that Iran cannot absorb further sophisticated systems.

The seven-page letter was signed by 56 lawyers, writers, professors, judges and former members of the suppressed National Front that opposed the shah in the 1950s. It marked the boldest move yet in a growing middle-of-the-road dissident movement that demands greater freedom and political expression without actually calling for abolition of monarchy.

Calling for the release of all political prisoners, the letter seeks dissolution of the rubber-stamp Parliament, return to constitutional government and reinstatement of an independent judicial system.

While not mentioning Shah Mohammed Reza Pahlevi by name, the signers appeal to Iranians to "raise their voices" against dictatorship and demand the prosecution of all violators of "constitutional rights." Among the signatories are Fereidoun Adamiyat, a historian and former ambassador to India, Ahmad Sdar, a judge and former Tehran prosecutor, and Karim Sanjabi, an ex-law professor and National Front activist who was imprisoned several time in the 1950s after the coup that toppled leftist Premier Mohammed Mossadegh and returned the shah to power.

Iranian interest in new weaponry from the United States remains high despite the Carter administration's announced intention to restrict arms sales abroad and congressional calls to halt further sales to Tehran.

Among items the shah is expected to discuss during his visit are additional advanced radar picket planes, late-model fighters, navy patrol boats and miliary transport planes.

The shah arrived today in Paris, where he will be joined by his wife, Empress Farah, who has been visiting Belgium. The couple is to visit Williamsburg on Monday, before the official part of the U.S. visit Tuesday and Wednesday, sources said. Iranian officials have refused to reveal the exact schedule in an effort to avoid anticipated demonstrations by dissident Iranian students.

Informed sources siad the administration's arms policy toward Iran would be acmain issue to be raised by the shah in his talks with President Carter, who will be the eighth U.S. president the Iranianlmonarch will have met during his 36-year reign.

High on the Shah's list will be up to three additional Boeing airborne Warning and Control System (AWACS) radar planes,' in addition to seven approved for sale to Iran last month when Congress declined to block the $1.2 billion deal.

Gen. Hassan Toufanian, vice minister of war and arms procurement chief also said talks are under way fro 140 General Dynamics F-16 fighters, adding to 160 already ordered for deliver starting in 1981 at an estimated cost of $3.8 billion.

The Shah is also expected to discuss replacements for his aging fleet of 221 F-4 Phantoms, sources here said.These planes will reach the end of their service life in the early 1980s.

The Shah has already requested permission to buy 250 F-18L fighters worth about $2.5 billion to replace his Phantoms. The proposal has run into congressional opposition because the plane is still on the drawing boards and has not been ordered by the U.S. armed forces.

"Certain elements in the Pentagon" have been urging Iran to buy F-15 fighters to replace the Phantoms, the sources said.

"Although it has not been finalized, the Shah has also indicated he wants to start talking about replacements for his F-5S, one insider said, adding that the present aim for this longer-term project is to acquire 160 planes. Iran has 160 F-5.

The Shah may also discuss procurement of up to six Navy patrol ships and at least 100 military transport planes "no replace and augment Iran's C-130 airlift fleet," an official said. "The Shah wants mobility to be able to move troops over great distances, for example southeast to Baluchistan." Baluchi tribesmen have been agitating for autonomy in recent years.

The arms requests are expected to meet opposition in Congress. When the deal for seven AWACS made it through Congress, Senate majority leader Robert Byrd (D.-W.Va.) called for a moratorium on further arms sales to Iran.

"Any furhter sales to Iran of AWACS, F-16s or F-18s would not be easily approved," a U.S. official said here.

Iran's total military acquisitions from the Untied States in the last nine years amount to some $18 billion, another official said.

The shah's basic rationale for the arms buildup is that Iran faces potential threats from every direction, cannot count on U.S. support in the even of an attack and must defend oil and gas reserves valued at more than $800 billion. "What does it matter if we spend $18 billion to protect $800 billion?" Iranians ask.

Leaders of Persian Gulf states officially deny that the buildup worries them, but privately some admit concern. Diplomatic sources report, for example, that Saudi Arabia once requested a U.S. intelligence readout of Iran's military aims.