BEIRUT, Aug 29 (Reuters) - Lebanon's top Christian cleric denounced on Sunday a bid to keep the Syrian-backed president through a constitutional change he said was "plotted by night" and forced on Lebanon's cabinet.

Maronite Patriarch Nasrallah Sfeir, a vocal critic of Syria's influence in Lebanon, opposes President Emile Lahoud's intention to remain in office after his term ends in November, which would require changing the constitution.

A hastily convened cabinet requested such a change to extend Lahoud's term by three years on Saturday, despite opposition from the prime minister and prominent politicians, including some traditional allies of powerful neighbour Syria.

"What happened yesterday regarding the constitution and the presidency is unfamiliar, plotted by night and carried out swiftly by day," Sfeir said during mass at the church's seat in the mountains of northern Lebanon.

"Those directly involved were seized to express a view imposed on them, and obeyed submissively," he said. "I call on all to be aware...and for God to help Lebanon and the Lebanese."

Syria flooded Lebanon with troops during the 1975-1990 civil war, later firming its grip through broad influence in the presidency, military and security services.

But his objections have been echoed by politicians closer to Damascus including Druze leader Walid Jumblatt, whose allies in the cabinet rejected the plan.

The U.S. State Department said last week the presidency should be a Lebanese rather than a Syrian choice, and determined according to Lebanon's constitution, a position repeated by German Foreign Minister Joschka Fischer in Damascus.

Washington is pressuring Syria to cut support for Lebanon's Hizbollah and pull its troops out of the country.

Syrian Foreign Minister Farouq al-Sharaa, hosting Fischer on Saturday, defended the move to retain Lahoud in office, saying respect for a country's constitution did not rule out the possibility of changing it.