In a surprise move, the directors of International Telephone & Telegraph Corp. tonight ousted Lyman C. Hamilton Jr. as chief executive of the giant multinational conglomerate.

In a statement, ITT said that Hamilton had resigned because of "policy differences" with the board.

Analysts said Hamilton's removal probably reflected a reassertion of authority by former chief executive Harold Geneen, whom the board forced out as chief executive in 1977, although he remained chairman.

Hamilton, who is a member both of the board and its executive committee, did not attend today's meeting, which lasted until after 8 p.m. He was the board's pick in 1977, after Geneen unsuccessfully fought to retain his chief executive status until the termination of his contract in May 1980.Geneen will be 70 then.

Hamilton will be replaced by Rand V. Araskog, 47, senior executive vice president and chief operating officer.

Araskog has been a favorite of Geneen's but Geneen had amicable relations with Hamilton as well, at least until Hamilton formally succeeded Geneen as chief executive on Jan. 1, 1978.

Since then Hamilton has presided over a reduction in some of the operations that Geneed added to the company during his decades of stewardship, including some European food operations.

Company sources report that Geneen as chairman and Hamilton as chief executive often have fought at board meetings over whether the company should get rid of operations Geneen had added to the empire.

The board and Geneen began to have serious differences in the early 1970s, after scandals surfaced surrounding ITT's attempted takeover of the Hartford Fire Insurance Co. and the company's cooperation with the CIA in overthrowing Chilean President Salvador Allende. Those differences finally led the board to force Geneen out as chief executive.

The company announced earlier today that it anticipated its second quarter earnings would be down about 20 percent from last year, mainly because of the weakening of the dollar in late June. Although ITT is based in the United States, it does most of its business overseas.

The company said that before taking foreign currency transactions into account, however, operating earnings were up about 10 percent over the second quarter of 1978.

Analysts said Hamilton's ouster probably has little to do with second quarter earnings.

"When you have a big multinational company, foreign exchange transactions are going to help you sometimes and hurt you other times," one analyst noted. CAPTION: Picture, LYMAN C. HAMILTON JR. . . . out at ITT