On the surface, two events Thursday signaled a breakthrough for Lebanon: * Armed with a package of reform proposals, Lebanese President Amin Gemayel traveled to Damascus for consultations with President Hafez Assad.
* Back home, warring militias began the first phase of a disengagement plan.
The oft-postponed summit started after Mr. Gemayel met Syria's conditions for specific progress on both the military and political fronts, to avoid a repeat of the failure at last month's peace talks. The high-pressure tactic appears to have led to an outline for changes that could even the balance of power between Christians and Muslims, as well as a timetable for a small pullback of rival factions so they are no longer in visual range of each other.
Although the prospects for peace seemed to increase, Voice of Lebanon radio described the mood in Lebanon as one of ''dim optimism.'' Syrian Vice-President Abdel Halim Khaddam called the meeting the ''last chance'' summit, and warned that if it failed, ''Syria would close down the Lebanese fire.''
That threat was backed up by Syrian Defense Minister Mustafa Tlas, who told a Kuwaiti newspaper, ''If asked by the Lebanese government, Syria has no objection to redeploying its forces in Beirut.'' This was interpreted as a reminder that if Lebanon does not arrange peace, Syria is prepared to use force to do it.
The Gemayel-Assad meeting is aimed at paving the way for the formation of a national-unity government, including representatives of all major factions, to replace the lame-duck Cabinet that resigned in February. It would then be charged with transforming the outline of reforms into constitutional changes.
Although there is no official word, the proposals are reported to be a revised version of a plan worked out in 1976 between Mr. Assad and then President Suleiman Franjieh. It was Mr. Franjieh's objections to weakening the Christian presidency that led to the impasse at the peace talks.
The Gemayel paper does, however, call for lessening the dominance of the presidency by spreading executive powers among a new ''council of ministers'' that would include the president, a Muslim prime minister, and a religiously-balanced cabinet, Lebanese sources say.
It also would divide parliamentary seats on a 50-50 basis between Christians and Muslims, ending the 6-5 edge allotted minority Christians since the country's independence 40 years ago. Confession-alism - distributing posts on the basis of religion - would be abolished from the civil service. The Christian-dominated administration would be decentralized.
The fragile nature of progress was evident in the latest cease-fire. As is the norm in Lebanon, the initial reaction to the ''definitive'' truce at 9 p.m. Wednesday was a nightlong battle. It did not ebb until just before Gemayel left for Syria.
The ability of the new truce-monitoring force to hold down the clashes, particularly by the small factions not included in negotiations, will serve as the litmus test for the Syrian-sponsored peace effort.
Small groups of French and Lebanese observers began taking up posts Thursday along the ''green line'' that divides the Christian and Muslim halves of the capital. On Friday a 2,000-man force under the control of Lebanese gendarmes is scheduled to begin deployment in the narrow buffer zones between the rival groups. However, all sides admit the force is too underarmed and understaffed to stop any serious truce violation. Instead it provides mainly a psychological barrier.
The potential for sabotage was evident Wednesday night when a grenade hit the west Beirut home of Druze leader Walid Jumblatt. Although the opposition leader has not lived there since his self-imposed exile after an assassination attempt in 1982, the building is still used as a local headquarters. It is just this kind of incident, comparatively small-scale, which spoilers could use to spark new fighting.
The bigger concern remains the Christians' position on their hold on power. Their hard-line stance has not eased since the Lausanne talks adjourned last month.
Former President Camille Chamoun said he was not prepared to accept any of the reforms the Christian parties had rejected in Lausanne. Fadi Frem, chief of the Christian Lebanese Forces militia, said Wednesday: ''We are against a dialogue with Syria on the Lebanese crisis.''
An official of the Christian Phalange Party said Thursday his party would not accept any national-reconciliation deal ironed out by Syria.
Although Gemayel, with Syrian help, appears to have worked out a plan that may meet opposition demands, it may cost him his already ebbing Christian support. In the end, his leadership and prospects for peace could be in as much trouble as before the Damascus summit.