Under the watchful eyes of heavily armed U.S. marines, Secretary of State George P. Shultz today continued his search for a Lebanese peace agreement in this capital where the terrorist bombing of the American Embassy 10 days ago killed nearly 50 persons.

"It's simply incredible to see such enemies of peace," a grim-looking Shultz said after inspecting the wreckage of the seafront embassy.

"But, at the same time, it's also inspiring to see that peace has friends who rally and help," he said in reference to his hope of getting Israel and Lebanon to agree on a plan for withdrawal of Israeli forces from this country.

The secretary then plunged into six hours of talks with Lebanese President Amin Gemayel and his top advisers about how to overcome the disagreements that leave the withdrawal negotiations stalemated four months after they began.

As in his discussions with Israeli Prime Minister Menachem Begin in Jerusalem yesterday and again this morning before flying to Beirut, Shultz stressed that his purpose in this initial phase of his shuttle mission was primarily to listen to views of the two sides.

Before returning to Jerusalem tonight, he said, "I can see there are quite a number of difficult issues. At least we now have some sense of them. . . . I still think it can be done. But I can't tell you how yet."

Shultz said he will continue his discussions with Begin on Friday and probably come back to Beirut on Saturday for another session with Gemayel. U.S. officials said that in these upcoming talks, the secretary was likely to begin putting forward his ideas about how to meet Israel's demands for security arrangements in southern Lebanon without violating Lebanese sovereignty.

The principal obstacle in the talks centers on Israel's insistence that chief responsibility for policing a security zone in southern Lebanon to prevent Palestinian raids into Israel be assigned to Christian militia forces commanded by Saad Haddad, a former Lebanese major allied with Israel.

Heavy security was evident from the moment that Shultz's plane landed at the Beirut airport. The plane immediately was surrounded by troops and armored personnel carriers from the 1,200-man U.S. Marine contingent here. At the embassy, the roofs of neighboring buildings were manned by marines and Lebanese soldiers, and machine-gun emplacements dotted the streets.

Lebanese state-run television said that some shells fell within a mile of the presidential palace soon after Shultz left there as fighting erupted between Christian Phalangist and Moslem Druze militias, United Press International reported.