DUBAI (Reuters) - Bahrain authorities prevented the wife of a Bahraini dissident and their infant son, a U.S. citizen, from boarding a flight to London after he staged a protest against a visit by the Gulf state's king to Britain, human rights groups said.

Sayed Ahmed AlWadaei, director of advocacy at the UK-based Bahrain Institute for Rights and Democracy, was one of two activists briefly detained by police in London when they shouted at the king's car as it approached Prime Minister Theresa May's office on Wednesday.

Hours later, the exile's Bahraini wife, Duaa, 25, and their 19-month-old son were detained for seven hours at Bahrain airport as they tried to fly out of the kingdom to join him in London, rights groups said.

In a statement, the Bahrain government said it briefly detained Duaa AlWadaei for questioning and a search, saying the king's presence in Britain and the discovery in Bahrain in recent times of weapons and explosives meant "precautionary security measures were necessary".

UK legal charity Reprieve said Duaa AlWadaei had been banned from leaving Bahrain, even though she had UK residency, adding Bahraini police and members of the public threatened Sayed Ahmed AlWadaei and his family in recent days.

In brief remarks to Reuters, AlWadaei said it was not clear to him for how long his wife would be prevented from leaving.

But the government said without elaborating that after the questioning, Duaa AlWadaei had been released "to make her onward destination".

"With regards to the specific allegations of mistreatment, at no time was Mrs AlWadaei subjected to any form of mistreatment or violence. Allegations that her child was also 'detained' are false; he was simply allowed to remain with his mother."

Bahrain's Sunni Muslim-led government has come under criticism from its Western allies and rights groups for its handling of dissent. Activists say a major crackdown is under way targeting Shi'ite and secular opposition.

Bahrain's Shi'ites say they suffer discrimination, though the government denies this. Manama accuses Shi'ite power Iran of fomenting unrest on its soil, a charged Tehran denies.

AlWadaei lives in exile in London with his wife and child. His wife and son were on a short family visit to Bahrain when the incident took place, Reprieve said.

The U.S. State Department said on Friday it was looking into reports that a child with U.S. citizenship was prevented from leaving Bahrain, according to the State Department website.

King Hamad, a close ally of Britain, was making his first visit to the country since May became prime minister in July.

(Reporting by William Maclean; Editing by Alison Williams)