Security forces reinforced by pro-government mobs fired rubber bullets and tear gas Friday to scatter protesters near Bahrain's royal palace, witnesses said, as conflict deepened between Sunni Muslims backing the ruling system and Shiites demanding it give up its monopoly on power.
The clashes broke out after an hours-long standoff between tens of thousands of demonstrators facing down lines of riot police and Sunni vigilantes carrying swords, clubs, metals pipes and stones. One protester, Habib Ibreeq, said people used private cars to ferry the injured to hospitals.
The latest clash reinforces the sense that nearly a month of protests led by the Shiite majority to demand sweeping political reforms was veering toward sectarian street battles between the divided communities. Shiites, who complain of discrimination, are also increasingly calling for the ouster of the Western-allied Sunni monarchy ruling the small but strategic island nation that is home to the U.S. Navy's 5th Fleet.
U.S. Defense Secretary Robert Gates, meanwhile, was in Bahrain on a mission to encourage dialogue between the protesters and the country's monarchy, Pentagon press secretary Geoff Morrell told reporters.
More than 700 people were treated for breathing problems and other troubles linked to tear gas, hospital officials said. Several others were hit by stones or cut by blades.
Some main opposition parties had called for the march to be canceled, fearing Bahrain was moving dangerously close to full-scale sectarian battles after weeks of protests modeled on the uprisings in Egypt and Tunisia. On Thursday, students clashed at a school and Sunni groups burned a Shiite-owned supermarket and threatened other businesses.
But Shiite youth groups ignored the appeals to call off the protest near the offices and compounds of Bahrain's king and other members of the ruling dynasty that has held power for more than two centuries.
The brief _ but intense _ melee began as protesters began to withdraw from a razor wire barrier separating the two sides. Witnesses said some stones were thrown from the pro-government mobs and they began to pour through an opening in the blockade. Within moments, police had fired tear gas and rubber bullets to drive back the demonstrators, according to Sayed Yousef al-Mahafdha of the Bahrain Center for Human Rights, who witnessed the clash. The use of rubber bullets was also witnessed by two protesters and Dr. Ali al-Iqri, who was part of an ambulance crew treating the wounded.
A statement earlier by Bahrain's Interior Ministry warned against holding the march amid a "level of sectarian tension that threatens Bahrain's social fabric."
Hours before the clash, pro-government bands attacked several cars trying to reach the area near the royal compounds.
Johnny Miller, a British cameraman on assignment for Iran's state-run Press TV, said dozens of assailants broke the windows of their car and insulted his Bahraini Shiite assistant.
Many Shiites in Bahrain claim the pro-government mobs include Sunnis from other Arab states and South Asia who are given citizenship and jobs under a government program to try to offset the Shiites' demographic advantage.
"This is a systematic operation to unleash these thugs to threaten Shiites and act as enforcers for the ruling system," said a prominent human rights lawyer, Mohamed al-Tajer.
Major Sunni-Shiite clashes occurred during the 1990s and forced Bahrain's Sunni rulers to introduce political reforms that included an elected parliament. But the island's Shiites _ about 70 percent of the population _ still see themselves stuck in a permanent underclass status.
They are effectively blackballed from top government or security posts and complain that voting districts are gerrymandered to prevent a Shiite majority in the 40-seat parliament, where the main Shiite bloc took 18 seats in elections last year.
A main grievance is the Sunni naturalization policies, which seek to offset the lopsided Shiite population advantage and bulk up the ranks of loyalists. Opposition groups estimate tens of thousands of Sunnis from across the Arab world and South Asia have been brought to Bahrain in recent years.
On Wednesday, thousands of Shiites marched outside the immigration office in the capital, Manama, to decry the "political naturalizations" and demand a mass expulsion.
Bahrain's leaders, meanwhile, are under strong regional pressure to stand firm.
The other Sunni Arab dynasties in the Gulf _ particularly Saudi Arabia _ fear any crack in Bahrain could encourage more uprisings across the oil-rich region to demand an end to their authoritarian grip. Protests have flared already in Oman and Kuwait. Saudi security forces were out Friday in a major show of power amid rumblings of wider demonstrations.
The Gulf Sunni leaders also see Bahrain as a potential beachhead for Shiite powerhouse Iran. Although there is no evidence of political ties between Tehran and Bahrain's main Shiite groups, some hard-liners in Iran have called Bahrain the "14th province" of the Islamic Republic.
On Thursday, the six-nation Gulf Cooperation Council backed a $20 billion aid plan for Bahrain and Oman, the least well-off in the super-wealthy bloc.
In Geneva, U.N. human rights officials said three prominent human rights activists in Bahrain are being targeted by death threats conveyed through Facebook and other social media sites. Rupert Colville of the U.N. Office of the High Commissioner for Human Rights said the messages on Facebook and other social media websites denounce the three men as "traitors" and aim to incite people to kill them.
The three being targeted are Mohammed Al Masqati, president of the Bahrain Youth Society for Human Rights; Naji Fateel, another member of the society; and Abdulhadi Alkhawaja, a former director at Frontline Defenders, Colville said.