Manama, Bahrain (CNN) -- The arrest in Bahrain of several prominent opposition figures has left the United States "deeply troubled," according to a statement Friday from the State Department.

"We are particularly concerned about the arrest of Ibrahim Sharif, a prominent leader of Wa'ad -- a large, registered, legitimate political society recognized by the government of Bahrain, as well as the reported detention of Dr. Ali Al-Ekri, a senior physician at Bahrain's largest public hospital, Salmaniya Medical Center," the statement said.

The United States called on the Bahraini government to ensure the security of the detainees, and to end violence against protesters.

Amnesty International also condemned the detention of the opposition figures and called for their immediate release. Eight activists were detained Thursday, according to a media release from the rights group. The release says they were arrested at their homes in armed raids conducted by a joint force of Bahraini and Saudi Arabian troops, that no arrest warrants were produced and that authorities have not allowed the detainees access to lawyers.

At a news conference Friday, Bahrain's Foreign Affairs Minister defended the decision to request the assistance of Peninsula Shield Forces from the Gulf Cooperation Council, of which the Saudi Arabian troops are members. The United Arab Emirates also sent troops as part of the GCC force.

The arrival of the troops is in accordance with international law, Foreign Minister Sheik Khalid bin Ahmed bin Mohammed Al Khalifa told reporters in Manama on Friday. They were sent in order to maintain security and stability and should not be seen as foreign interference in Bahrain's internal affairs, he said.

Meanwhile Friday, two doctors from Salmaniya Hospital said security forces were "beating patients."

The unnamed doctors said "six to 10 patients" were removed from the facility, but they did not know where the patients were being taken.

Security forces were also accused of "taking any evidence of presence like medical records and x-rays."

CNN could not independently confirm this information but has contacted the government for comment.

On Thursday, U.N. High Commissioner for Human Rights Navi Pillay expressed concern at reports that security forces were targeting hospitals. She said her office had been receiving "desperate calls" regarding, among other things, "the takeover of hospitals and medical centers by various security forces," which reportedly include troops from Peninsula Shield Forces.

Also Friday, security forces in Bahrain demolished the Pearl Monument, a landmark that had been the site of massive recent anti-government protests.

The government explained the demolition by saying it was done "out of the government's keenness to optimize services and improve the infrastructure" and that it would "boost flow of traffic in this vital area of the capital," according to the state-run Bahrain News Agency.

Pieces of the demolished modern white structure lay like a pile of bones, according to pictures broadcast on Bahrain state television.

Thousands of people congregated at the Pearl Roundabout during the height of anti-government demonstrations last month. The highway leading to the roundabout was clogged with protesters this week, though it was clear Friday and streets were quiet after a government crackdown on protesters.

Members of the opposition expressed their disappointment at the demolition of a structure the government itself had described as one of the kingdom's "most striking landmarks."

"I think it's a stupid act from the authorities that they think by removing this it will be removed from history," said Khalil Al Marzooq, a leading member of the Wefaq party. "But unfortunately what they have created is a historical feeling and records that will not be wiped out. They will continue to be rulers of this country by the tanks and Saudi troops, not by the feeling of the people."

Troops from Saudi Arabia and the United Arab Emirates moved into Bahrain on Monday, the Bahraini government said. The troops arrived under the banner of the Gulf Cooperation Council, an association of six Gulf Arab states.

Al Khalifa said their presence is simply to protect the country's stability and security.

"The view is quite clear," he said in an address on national TV. "The urgent matter is to restore the security and stability. How can we start the dialogue without having security and stability?"

CNN's Leone Lakhani contributed to this report.