U.S. fighter jets dropped dummy bombs on the Navy's prized firing range on Vieques island Monday, resuming exercises despite protests seeking a halt to training on the island.

The practice bombing runs began about 2 p.m., Navy Lt. Cmdr. Katherine Goode said. Activists opposed to the exercises had invaded the range and reportedly were hiding there even as the exercises began.

Protest leader Ismael Guadalupe said the bombing posed "a threat to human life."

Navy officials, however, said there were no protesters remained on the bombing range on the eastern tip of the Puerto Rican island.

Eight demonstrators were arrested Monday morning for trespassing on Navy property as they walked toward the range, said Navy spokesman Bob Nelson.

The Navy had said fighter jets from the USS Theodore Roosevelt aircraft carrier could begin dropping inert bombs as early as 8 a.m. Monday. But bombing began about six hours later, and protesters claimed victory in forcing the delay.

Nelson denied that protesters delayed the exercises. He said bombing had not started in the morning because the Navy still was planning and coordinating the exercise.

Protesters said as many as 30 anti-Navy activists were on the range.

"The people of Vieques and Puerto Rico have been able to fool all the security forces, the police officers and the radars guarding the range," protest leader Nilda Medina said.

On Sunday night, demonstrators threw rocks at Navy security officers along the fence surrounding the Navy's Camp Garcia, Nelson said. One sailor was hit in the leg with a rock but was not seriously injured.

President Bush announced last week that the Navy would leave Vieques in two years, but protesters said that was not good enough.

Jacqueline Jackson, wife of the Rev. Jesse Jackson and co-president of his Rainbow/Push Coalition civil rights group, joined the protests Sunday night, speaking to more than 200 demonstrators. She told the protesters she would participate "however you can use me for your purpose."

More than 180 people were arrested for trespassing during the last exercises in late April and early May. They included environmental lawyer Robert Kennedy Jr., actor Edward James Olmos and the Rev. Al Sharpton, who has been on a hunger strike in a New York prison since May 29.

The Navy says Vieques' terrain and location make it ideal for combat training. The island also is strategically located southeast of the U.S. mainland and off the 22,600-acre Roosevelt Roads Naval Station, the largest in the world in land area, which administers the Vieques exercises.

Opposition to the exercises swelled in 1999 after off-target bombs killed a civilian guard on the bombing range.

Bush's announcement of a withdrawal within two years appeared to pre-empt a planned referendum Nov. 6, when most of the island's 9,100 residents were expected to vote for the Navy to leave in 2003.

But Puerto Rican officials are planning to hold their own nonbinding referendum on Vieques on July 29 asking residents whether the Navy should stay, leave in 2003 or leave immediately.

Puerto Rican Gov. Sila Calderon has demanded an immediate end to the exercises, saying the bombing harms the environment and islanders' health. The Navy denies the charges.

During a news conference Monday with the other service secretaries, Navy Secretary Gordon England reiterated the Defense Department's plan to stop Naval training on Vieques by 2003.

"We're actively planning to leave by 2003," he said. "I sincerely believe we will have an alternative place by that time."