BEIRUT (Reuters) - Lebanese riot police fired smoke grenades and sprayed water on Saturday to disperse dozens of students protesting against the visit of senior U.S. diplomats to Beirut.

The protest turned nasty when security forces tried to clear protesters who gathered outside the government headquarters ahead of a visit by U.S. Assistant Secretary of State David Welch.

Some of the protesters, waving Lebanese flags and carrying placards protesting against U.S. influence in Lebanon and the Middle East, pelted police with stones.

"Welch is not welcome in Lebanon," one placard read.

Welch, who met with several Lebanese officials on Saturday, was due to hold talks with Lebanese Prime Minister Fouad Siniora at the government headquarters.

Welch and U.S. Deputy National Security Adviser Elliott Abrams began a trip to the region last week.

Their visit had been postponed because of Israeli Prime Minister Ariel Sharon's stroke but the United States has resumed efforts to resolve the Israeli-Palestinian conflict without him.

The United States is assailed in much of the Arab world for what many perceive as its pro-Israeli stance.

But Washington has pledged to support Lebanon since Syria pulled its forces out of its smaller neighbor in April amid intense international and local pressure following the killing of Lebanese former Prime Minister Rafik al-Hariri.

A UN inquiry has already implicated Syrian officials in the murder. Damascus denies any role.