Premier Adolfo Suarzez, architect of Spain's post-Franco transition to democracy, today tendered his resignation to King Juan Carlos. The unexpected move created a major political crisis here.

Initial speculation was that rightist military pressure could have provoked the resignation of the 48-year-old premier, who had held office since June 1976.

Juan Carlos must now consult with political leaders and propose a candidate from the ranks of Suarez's Union of the Democratic Center party, which won the last general election in 1979. The candidate must then gain majority backing from the 350-member congress, where the party holds 167 seats. In the meantime, the Suarez Cabinet continues as a caretaker executive.

In a nationwide broadcast, Suarez gave no specific reason for his resignation other than that he had been the victim of constant personal attacks. He said no one had asked for his resignation.

"There are times when it is more beneficial to the notion to the remain," he said.

Urging the nation to stand behind his successor, Suarez gave no indication who that might be. He promised to remain as a congressman.

Political sources said the resignation could well have been caused by a buildup of pressure among conservative elements in the armed forces who radically oppose alleged secret negotiations between the Suarez government and Marxist Basque separatists of the ETA organization.

Suarez had faced increasing criticism for his handling of the terrorist problem, responsible for 120 deaths last year, and the stagnant economy with an unemployment rate close to 12 percent. In recent midterm by-elections and regional assembly votes, his party suffered significant defeats. Polls showed a continued decline in Suarez's popularity to below 30 percent.

Shock over the resignation centered principally on its timing, which has left the ruling party at logger-heads. The public criticism of the Suarez government had spilled into the ranks of his party, which was to hold its biennial convention this weekend. The convention was postponed earlier this week when an air controllers' strike threatened to prevent many of the convention delegates from reaching the site in Majorca.

The party, which pursues a pro-Western centrist policy, was welded together by Suarez out of a coalition of Christian Democrats, Social Democrats, liberals and the more democratic sectors of the old regime under the late dictator Francisco Franco. The premier, who simultaneously resigned his post as party president, leaves no obvious successor.

The surprise of the timing is compounded by the coming political schedule of King Juan Carlos, which includes his first visit to the Basque country as monarch nezt week and an official trip to the United States Feb. 9.