The five, held since late 2001 or early 2002 along with more than 600 others suspected of fighting with the Taliban in Afghanistan or supporting al-Qaida, landed at a military air base on the outskirts of London.
"Four (of the men)... were arrested. Each man will be interviewed by officers from the Anti-Terrorist Branch," said police spokesman Peter Clarke.
If police decide there is no case against the men under Britain's anti-terror laws, they may be freed in days. The five were considered a low security risk by US authorities.
Police said the fifth man was detained at the RAF Northolt air base for questioning but had not been arrested.
Human rights controversy
The long detention of the men in Guantanamo without trial or access to lawyers has become a human rights cause celebre in Britain.
"It's about time they give my brother his life back," Sharon Fiddler, sister of returning detainee Jamal al-Harith, told said.
"The best thing for the government is for the word Guantanamo not to be mentioned"
Police said the four would be given medical examinations to ensure they were fit to be detained, and would be interviewed at a London police station.
They would be allowed to make a phone call and speak to a lawyer, police addded.
Three of the five were from the central English town of Tipton - dubbed Tipton Taliban by the media.
Relatives avoided waiting media, but a family friend of one of the men, Rhuhel Ahmed, said in a statement: "The family believe that if there were any evidence (he) had done anything wrong, the Americans would already have used it against him."
US authorities say that of 100 inmates released from Guantanamo, 88 were allowed to go free in their home countries, while 12 were kept in detention. Four Britons remain in Guantanamo.
A British police statement said two independent observers, one from the Muslim community, accompanied the five on the flight to Britain and the journey had been videotaped.
The men were not restrained during the flight and had been allowed to wear civilian clothes, the statement said.
Their treatment on their journey home was in stark contrast to past images of Guantanamo beamed around the world of shackled inmates being kept in cages.
Campaigners welcomed the return of the five, saying they had finally escaped the "legal limbo" of Guantanamo and should now enjoy full legal rights.
Prisoners are held in 'legal limbo'
at the US base in Cuba
While Prime Minister Tony Blair's supporters say the five's return shows the benefits of his close relationship with US President George Bush, analysts say Guantanamo is a no-win issue for the British government.
If the five were charged in Britain, it would signal the Americans were right to detain them and negate the perception London had righted a US injustice, they say.
If freed, people would want to know why it took so long to get the men home.
"The best thing for the government is for the word Guantanamo not to be mentioned," said analyst John Curtice.