Pro-Syrian Moslem and Druze opposition leaders and Greek Orthodox figures met in this Syrian-controlled town today to proclaim a National Unity Front and to demand changes in Lebanon's system of political representation, which now favors the Maronite Christians.

While many of the same politicians and factions have made similar demands in the past, to no avail, today's gathering, held under Syrian sponsorship, underscored the factional opposition to President Amin Gemayel, a Christian, and the breakdown of the political system by which Lebanon has been governed since 1943.

The new alliance, made up of 15 political parties and 30 independent politicians, issued a statement calling for a "democratic and secular" Lebanon, with wide-ranging constitutional and electoral reforms.

The purpose of the changes would be to abolish an outdated system of proportional government that favors the Maronite Christians and, to a lesser extent, the Sunni Moslems. The Shiite Moslems, once the smallest of the three communities, are now the country's largest but have not been able to gain a reapportionment of political power.

Druze leader Walid Jumblatt and Shiite Moslem leader Nabih Berri, both members of the coalition Cabinet, were the key political figures at today's meeting, which included participants from a variety of Syrian-backed leftist groups, such as the National Syrian Social Party, the Lebanese branch of the Baath Socialist Party and the Lebanese Communist Party.

Also present were leaders of Lebanon's Greek Orthodox Christian community, including House Speaker Munir Abu Fadel and banker Elias Saba. The Greek Orthodox have felt left out of Lebanon's main political process. They objected strongly last year that they were not adequately represented at a conference in Switzerland that attempted, without success, to bring about political changes similar to those proposed today.

Absent from today's deliberations were Sunni Moslem leaders from Beirut and former president Suleiman Franjieh, a Christian Maronite politician and foe of Gemayel. Franjieh and Elie Hobeika, leader of the Lebanese Forces Christian militia and also a rival of Gemayel, patched up a seven-year feud last week, however, further isolating Gemayel.

Berri lobbied strongly for shifting the presidential election from parliament, which is Christian-controlled, to a popular vote. Saba, who was finance minister in past governments, said that "the purpose of this meeting is to regroup Lebanon on a nonconfessional basis."

Some Sunni Moslem politicians who have figured prominently in Lebanese political life boycotted the conference. This was criticized by Tammam Salam, son of former prime minister Saeb Salam. He predicted failure for the front, saying it will be a repeat of the failures of the defunct Nationalist Movement, a coalition of Moslem and leftist groups that flourished during the 1975-1976 civil war. The Sunni Moslems have been unhappy with the political emergence of the Shiites.

Until today, Berri had avoided taking part in any fronts openly sponsored by Syria, studiously remaining aloof in his dealings with it. By today's move, in the view of analysts, Berri has reduced his freedom of movement and put himself under the influence of the Syrians. Syrian soldiers guarding the entrance to the hotel here used the butts of their rifles to prevent his bodyguards from entering.

In the statement proclaiming the National Unity Front, the signatories also called for the trial of "agents" dealing with Israel and of officials accused of "crimes against the population." Jumblatt often has called for the trial of Gemayel for his role in fighting between Lebanese Army forces and Druze militiamen in 1983 and 1984.