(CNN) -- Bahrain has foiled a foreign plot to destabilize it, the country's king said Sunday.
King Hamad said the plot had been in the making for more than two decades -- but did not name a country that he believed was trying to carry it out.
Bahrain's Sunni Muslim monarchy has long suspected Iran of attempting to foment unrest among the island's majority-Shiite population.
Relations have been tense in recent weeks as anti-government protesters have taken to the streets of Manama and Iran has condemned Bahrain's violent crackdown.
The king was speaking to officers from the Peninsula Shield, the military arm of the Gulf Cooperation Council. Several members of the council have sent troops to Bahrain to help Manama quell the protests.
"An external plot has been fomented for 20 to 30 years for the ground to be ripe for subversive designs," the king told the gathering.
"Such subversive designs are not however possible, whether in Bahrain or in any other GCC country, thank goodness," he added, according to a report by the Bahrain News Agency. "I here announce the failure of the fomented subversive plot."
King Hamad said that if the plot succeeded in one council country, it could spill over into others.
Meanwhile, Human Rights Watch urged Bahrain on Monday to end its "campaign of arrests" of doctors and human rights activists.
Over the weekend, security forces arrested four medical doctors and two activists, the group said.
"The arrests, some of which occurred during pre-dawn hours, appear part of a broader government crackdown involving nighttime raids on the homes of those viewed as supporting pro-democracy protesters," Human Rights Watch said.
Among those detained was Nabeel Rajab of the Bahrain Center for Human Rights.
About 25 people in about a dozen cars pulled up to his house early Sunday and took him to the offices of the interior ministry's investigative department, he said.
"They said that they were looking for a suspect who was armed and thought I might know him," Rajab said. "They beat me, punched me, kicked me, handcuffed me. Blindfolded me."
The government confirmed that Rajab was arrested but did not provide additional details.
In a statement released Monday, the government denied accusations from the Human Rights Watch suggesting "there is a campaign of indiscriminately arresting or targeting doctors."
"What such organisations have so far failed to understand is that the services of some of Bahrain's main medical facilities, including Salmaniya Medical Complex, had been overrun by political and sectarian activity," the government said in a statement.
"This was totally unacceptable behaviour, by any standard. "Those responsible are being investigated and will be held to account in the proper, legal manner."
Thousands of people have been demonstrating in Bahrain since last month, part of a wave that has spread through North Africa and the Middle East.
On Saturday, U.S. Secretary of State Hillary Clinton called on Bahrain to allow its people to demonstrate peacefully as opposition members reported the death of a fifth protester.
"We have made clear that security alone cannot resolve the challenges facing Bahrain," Clinton said. "Violence is not and cannot be the answer. A political process is. We have raised our concerns about the current measures directly with Bahraini officials and will continue to do so."
The demonstrators were killed when Bahraini security forces cleared protesters from the Pearl Roundabout in the capital, Manama, on Tuesday. The roundabout had been a rallying site for anti-government demonstrators since the unrest began.
Three days later, the government demolished the landmark monument at the center of the traffic circle.