By Lin Noueihed and Frederik Richter
MANAMA (Reuters) - Iran denounced the arrival of Saudi troops in Bahrain as unacceptable on Tuesday and the United States urged its nationals to leave the island, which has been roiled by a Shi'ite uprising against the Sunni elite.
Analysts saw Monday's troop movement into Bahrain, home to the U.S. Navy's Fifth Fleet, as a mark of concern in Saudi Arabia that concessions by the country's monarchy could inspire the conservative Sunni-ruled kingdom's own Shi'ite minority.
More than 1,000 Saudi troops rolled over the causeway that links it to the island kingdom and the United Arab Emirates also said it would send 500 police at the request of Bahrain's rulers after weeks of protests against the government and royal family.
Over 60 percent of Bahrainis are Shi'ites who complain of discrimination at the hands of the Sunni royal family. Calls for the overthrow of the monarchy have alarmed the Sunni minority, which fears that unrest could serve non-Arab Shi'ite power Iran.
Iran, which sits across the Gulf from Bahrain, sharply criticized the Saudi intervention.
"The presence of foreign forces and interference in Bahrain's internal affairs is unacceptable and will further complicate the issue," Foreign Ministry spokesman Ramin Mehmanparast said at his weekly news conference in Tehran.
The United States has urged Saudi Arabia, the world's top oil exporter and a key U.S. ally in the Gulf Arab region, to show restraint, though analysts said the escalation showed the limits of U.S. influence when internal security was threatened.
In a sign that security in Bahrain could deteriorate, the U.S. State Department advised against all travel to the island due to a "breakdown in law and order."
Bahrain has been gripped by its worst unrest since the 1990s after protesters took to the streets last month, inspired by uprisings that toppled the leaders of Egypt and Tunisia.
Unlike those countries, where the mainly Sunni populations united against the regime, Bahrain is split along sectarian lines, raising the risk of a slide into civil conflict.
Sectarian clashes broke out in different parts of Bahrain overnight, with both Sunnis and Shi'ites trading accusations in newspapers and on local television that they had been attacked by gangs of youths.
Clashes using clubs, knives and rocks have become daily occurrences, forcing Bahrain University and many schools to close in order to avoid further violence.
The intervention by Gulf Arab troops is highly sensitive in a region where tensions soared after the 2003 U.S. invasion of Iraq overthrew a Sunni dictator and ushered in a Shi'ite-dominated government.
Accusations already abound of Iranian backing for Shi'ite activists in Bahrain -- charges they deny.
Thousands of demonstrators are still camped out at Manama's Pearl roundabout, the focal point of weeks of unrest.
Despite some reports that the protesters planned to reopen a main thoroughfare to Bahrain's financial district at dawn, metal barricades and piles of sand and rocks still blocked the road. At checkpoints near the roundabout, activists checked identities and waved cars through.
In a sign that the opposition and the royals may find an 11th-hour solution, the opposition groups said they had met the crown prince to discuss the mechanism for national dialogue.
Even if talks are successful however, the opposition is increasingly split and hardline groups may keep up protests.
Bahrain's largest Shi'ite Muslim party, Wefaq, wants a new government and a constitutional monarchy that vests the judicial, executive and legislative authority with the people.
A coalition of much smaller Shi'ite parties is calling for the overthrow of the monarchy -- demands that scare Sunnis who fear this would benefit Iran.
(Additional reporting by Robin Pomeroy in Iran)
(Editing by Crispian Balmer)