By Stephen Eisenhammer

CURITIBA, Brazil (Reuters) - Twelve Brazilian suspects arrested for discussing a potential attack during the Olympic Games in Rio de Janeiro were "no joke," the prosecutor for the case told Reuters on Monday.

Dismissing criticism that the recent arrests were a calculated move to show Brazil taking the threat of terrorism seriously, Rafael Brum Miron said the suspects, alleged sympathizers of the Islamic State militant group, discussed an attack with at least two foreigners who formed part of a messaging group.

One suspect, he said, wrote via the Telegram messaging service that "the Olympics are an opportunity to reach paradise." Another, who was detained on Sunday following a search after initial arrests last Thursday, had discussed acquiring heavy weaponry.

Miron's comments follow some questioning of the operation because Brazil's justice minister, when announcing the arrests, called the suspects "absolutely amateur" and said they had no specific plans or capabilities to attack the Games that begin on Aug 5.

"They were amateurs, but they were not a joke," Miron said in his office in the southern city of Curitiba. "There's no such thing as an experienced suicide bomber."

Justice Minister Alexandre Moraes, who has faced criticism for minimizing the threat of an attack during the Games, said security agencies were watching anyone who visited websites that "make an apology for terrorism" even if they were just curious and not planning to do anything.

"Our police and intelligence agencies are continuing to monitor more than 100 people so that we can have tranquility during the Games," Moraes told reporters after inspecting security equipment at the Brasilia airport at the start of a tour of the country's main entry ports for visitors for the Games.

Moraes said he had instructed the high security prison in southern Brazil where the 12 men are being held to allow them immediate access to lawyers to guarantee their legal rights.

The U.S. Federal Bureau of Investigation gave Brazil the tip that led to the arrests, according to Miron, by sending a succinct report that said "these people merit investigating."

He said the last of those arrested, detained Sunday in the state of Mato Grosso, is actually a leader of the group. Officials initially played down his importance in order to prevent a panic at the possibility of an alleged militant on the loose, he added.

At least two members of a wider group of about 20 people investigators monitored were foreigners, Miron said, explaining that sometimes awkward messages from them appeared to have gone through Google Translate.

The foreigners, he added, are believed to be from Africa.

Brazilian police are now investigating possible connections with Islamist groups there.

As police scour seized computers and phones, initial evidence suggests that Brazil itself was not the target, but that the suspects saw the Games as such because "there are lots of people coming from abroad," Miron said.

No specific country was singled out for attack, nor was any date or target finalized, but investigators have said the group was interested in striking countries currently in conflict with Islamic State.

Miron would not specify how investigators intercepted messages via Telegram, a service that encrypts data between users. He said, however, it did not require help from the company.

In a separate interview televised Sunday, the judge overseeing the investigation said investigators had obtained data from Facebook Inc. and Twitter Inc. because the suspects had also used those platforms.

(Additional reporting by Anthony Boadle in Brasilia; Editing by Paulo Prada and Mary Milliken)