An explosion of shellfire has complicated President Reagan's apparent bid for a face-saving redeployment of Marines from Beirut, and Lebanese Druze leaders warned of ''revenge'' against remaining American ''diplomats, civilians, and interests'' in the city.
Druze leader Walid Jumblatt maintained that the new outbreak of violence had come even as he and Syrian leaders were weighing ways of bringing a United Nations force to Beirut to facilitate an orderly and ''honorable'' pullout of the Marines and their West European partners in Beirut's multinational force.
Diplomats here said Mr. Jumblatt is also understood to have received a message recently on the subject from the Soviet Union, which could veto any such UN move from its seat in the Security Council.
There was no word on the contents of the note - said to have come from Soviet Foreign Minister Andrei Gromyko after he had been queried on the UN option by European foreign ministers at the European security conference in Stockholm last month.
Earlier Wednesday, the Syrians and their Soviet allies announced that Kremlin Politburo member Geidar Aliyev would arrive here within the next week for talks with Syrian leaders. Mr. Aliyev will be the highest level Kremlin visitor here since Israel's 1982 invasion of Lebanon.
Tuesday night, Mr. Reagan said he was ordering the Marines redeployed to vessels offshore. He coupled this with a vow to unleash naval and air strikes against further fire on Beirut ''from parts of Lebanon controlled by Syria.''
But on Wednesday fresh shell exchanges erupted between Maronite Christian east Beirut and Druze positions in the hills above the capital - an area to which troops of the Syrians' 40,000-man force in Lebanon have free access. The US battleship New Jersey then boomed dozens of shells onto the Druze areas.
''You want to kill us,'' Syrian-supported Druze chief Jumblatt, who is in Damascus, protested to the US Embassy in a phone conversation late in the day. Jumblatt charged the naval bombardment centered on civilian targets in the hill town of Tibyat.
''If you want us to be terrorists, we will be terrorists,'' Jumblatt said he told the embassy.
A Jumblatt aide, Marwan Hamadeh, said the US shelling ''can have very serious consequences for American civilians, diplomats, and American interests'' in west Beirut, the part of the city captured by Druze and Shiite militiamen overnight Monday and the area where virtually all US diplomats and journalists live.
The lanky, mustachioed Jumblatt, toying with a string of white worry beads, declined to exclude civilians from his own warning of ''revenge'' - although he did say his main immediate concern was to cement control of west Beirut and prevent any counterthrust by the Lebanese Army.
The widespread assumption among Western analysts here was that Lebanon's Phalangist Christian milita - increasing unable to count on help from its one-time Israeli supporters, and convinced the Marines' redeployment was but a first step in a total pullout - started Wednesday's firing in hopes of holding Reagan to his pledge of renewed intervention.
But in Lebanese tradition, it was impossible to determine with certainty who had fired first.
In any case, no Arab or Western diplomat here doubted that the Marine redeployment - and pressure for Israeli disengagement from southern Lebanon - stood to benefit anti-Phalangist Muslim militias and their Syrian supporters.
A diplomat from a moderate Arab state lamented that the US redeployment, while not surprising given the forces' ever more precarious position, could not help but be a boon to radicals in the region, and impair US credibility with more moderate regimes.
Syria, sitting pretty, withheld immediate comment on Reagan's redeployment move. President Hafez Assad is also understood to have given no immediate reply to a request from Lebanese President Amin Gemayel for a face-to-face meeting on Lebanon's future.
The request was relayed Tuesday by a delegation of mainstream Sunni Muslim politicians from Lebanon - including three former prime ministers.
The delegation chief, Takieddine Solh, said Mr. Gemayel had told him before the group left for Damascus of a desire for ''good'' relations with Syria and a conviction that Lebanon's problems ''cannot be resolved without Syria's playing a major role.''
Mr. Solh said it was his impression that, at present, Mr. Assad did not share the conviction of Gemayel's main opposition - the Druze and Shiites - that he must resign.
Moreover, conversations with the Sunnis and Druzes here suggested neither group thought Gemayel would step down in the immediate future.
Some diplomats here did hold out the hope that, despite the renewed violence in Lebanon, the negotiated route to stability there might reopen in the days ahead.
But the main roadblock, they felt, was a lingering tug-of-war between Reagan and Assad.
The US President's statement Tuesday suggested he was intent that any Marine pullout not come with the appearance that Syria had driven the Americans out or that Damascus had forced a retreat on American pledges of support for Gemayel.
This preoccupation also may explain Washington's decision not to dispatch special Mideast envoy Donald Rumsfeld to Damascus - a trip widely expected to have come Tuesday.
Western and Arab analysts here felt that Syria was unlikely to be cowed by renewed US naval or air strikes. ''They'll wait the Americans out,'' said one Western diplomat, ''in the knowledge that sooner or later the Marines will have to leave altogether.''
Meanwhile, said an Arab political analyst here, ''Assad is waiting for America's approach on a political resolution of the crisis, assuming this is but a matter of time.''