MANAMA, Bahrain—Thousands of anti-government demonstrators cut off Bahrain's financial center and drove back police trying to push them from the capital's central square — shaking the tiny island kingdom Sunday with the most disruptive protests since calls for more freedom erupted a month ago.

Demonstrators also clashed with security forces and government supporters on the campus of the main university in the Gulf country, the home of the US Navy's Fifth Fleet.

The clashes fueled fears that Bahrain's political crisis could be stumbling toward open sectarian conflict between the ruling minority Sunnis and Shiites, who account for 70 percent of the nation's 525,000 people.

In some neighborhoods, vigilantes set up checkpoints to try to keep outsiders from entering. Bahrain's interior ministry warned Saturday that the "social fabric" of the nation was in peril.

Bahrain's Crown Prince Salman bin Hamad Al Khalifa said the nation has "witnessed tragic events" during a month of unprecedented political unrest. But he warned, "the right to security and safety is above all else."

"Any legitimate claims must not be made at the expanse of security and stability," Salman said in a televised speech late Sunday.

A day after visiting US Defense Secretary Robert Gates urged quick progress toward reform, thousands of protesters gathered before dawn to block King Faisal Highway, a four-lane expressway leading to Bahrain's main financial district in downtown Manama, causing huge traffic chaos during morning rush hour and preventing many from reaching their offices on the first day of the work week.

"No one was able to go to work today. Thugs and protesters were blocking the highway," complained Sawsan Mohammed, 30, who works in the financial district. "I am upset that Bahrain is no longer a stable place."

Security forces dispersed about 350 protesters "by using tear gas," the government said. But traffic was clogged until late morning, and many drivers sent messages of rage and frustration to social media sites.

"I blame the protesters for what's happened in Bahrain today," said Dana Nasser, 25, who was caught in the traffic chaos and never made it to her office.

About two miles (three kilometers) away, police at the same time moved in on Pearl Square, site of a month-long occupation by members of Bahrain's Shiite majority calling for an elected government and equality with Bahrain's Sunnis.

Many protesters in recent days have pressed their demands further to call for the ouster of the Sunni dynasty that has held power for more than two centuries.

Witnesses said security forces surrounded the protesters' tent compound, shooting tear gas and rubber bullets at the activists in the largest effort to clear the square since a deadly crackdown last month that left four dead.

Activists tried to stand their ground and chanted "Peaceful! peaceful!"

The crowd swelled into thousands, with protesters streaming to the square to reinforce the activists' lines as police continued firing tear gas. By early afternoon, police pulled back from the square, witnesses said.

At Bahrain University, Shiite demonstrators and government supporters held competing protests that descended into violence when plainclothes pro-government backers and security forces forced students blocking the campus main gate to seek refuge in classrooms and lecture halls, said Layla al-Arab, an employee at the Arts Collage.

Two protesters sustained serious head injuries and hundreds sought medical help, mostly with breathing problems from tear gas, hospital officials said on condition of anonymity because they were not authorized to speak to reporters.

The Gulf kingdom holds particular importance to Washington as the host of the Fifth Fleet, the main American military counterweight to Iran's efforts to expand its armed forces and reach into the Gulf.

Bahrain has also tried hard to position itself as an attractive investment destination and Middle East banking center. Even the passport stamps issued to incoming visitors declare the kingdom as "Business-friendly Bahrain." With reports from Barbara Surk and Adam Schreck, AP