By Lisandra Paraguassu and Anthony Boadle

BRASILIA (Reuters) - Brazil arrested 10 people on Thursday suspected of belonging to a poorly organized group supporting Islamic State (IS) and discussing terrorist acts during the next month's Olympic Games in Rio de Janeiro.

The group, described as "absolutely amateur" by Justice Minister Alexandre Moraes, were all Brazilian citizens and in contact via messaging apps such as WhatsApp and Telegram. They did not know each other personally, the minister said.

The arrests came a week after a truck massacre in Nice, France, and amid growing fears of a possible attack when the first Olympics to be held in South America kicks off on Aug. 5. Some 500,000 visitors are expected to travel to Brazil for the Games, many of them from the United States.

Although Brazil has no history of conflict with known militant groups, Moraes said the Games had made the Latin American country a more likely target, particularly because of participation by countries fighting IS.

"Today was the first operation against a supposed terrorist cell in Brazil," he told a news conference. "Brazil was not part of the coalition against IS but, because of the upcoming Olympics and because it will receive many foreigners, Brazil has become a target."

Moraes said the individuals detained on Thursday were being monitored because they had accessed websites linked to IS, but the group had "no preparation at all" and was a "disorganized cell". He said that authorities intervened when the group started planning actions including martial-arts training and the purchase of firearms.

While the group did not have direct contact with IS, some members had made "pro forma" declarations of allegiance to the militant Islamist group via social media, the minister said.

A presidential aide said on Thursday that Brazilian police and intelligence services had cooperated with French, German, British, Israeli and U.S. intelligence agencies in the arrests.

Story continues

Brazil's federal police are monitoring around 100 people for possible links to terrorist groups, mostly in the lawless tri-border region with Paraguay and Argentina, said the aide, requesting anonymity.

"Brazil was really being preemptive in this case," said Scott Stewart, vice president of tactical analysis at Stratfor, a global intelligence and advisory firm. "There are some tactical benefits to arrests like this...You could disrupt anything in the works and it may also give other people pause."

OPERATION HASHTAG

Brazil plans to deploy about 85,000 soldiers, police and other security personnel during the Rio Games, more than twice as many in place for the London Olympics in 2012.

It will also operate a joint security center where representatives from more than 100 countries are expected to help share intelligence and monitor the event.

Local media raised concern that the arrests might affect ticket sales for the Games, with the release of 100,000 tickets on Thursday aimed at improving attendance.

The local Olympic organizing committee, Rio 2016, referred requests for comment to the federal government.

Moraes said there was no evidence that the group had acquired weapons though one member had contacted a clandestine weapons site in neighboring Paraguay that sells AK-47 assault rifles.

"Those involved participated in an online group denominated 'the defenders of Sharia' and were planning to acquire weapons to commit crimes in Brazil and even overseas," Moraes said.

Two people will be brought in for questioning, including the head of a local NGO, in addition to the 10 people arrested across as many states, Moraes added.

Federal Judge Marcos Josegrei da Silva, who approved the arrests from the southern city of Curitiba, said police had 30 days to analyze evidence. He emphasized that no-one had been charged in the operation, codenamed Hashtag.

"These are internet affirmations on social networks," Josegrei told a news conference. "Not everything a person conceives of in the virtual world is carried out in the real world."

Interim President Michel Temer called an emergency cabinet meeting following the arrests, the first under Brazil's tough new anti-terrorism law approved this year.

Brazilian and U.S. officials have raised concern over the disappearance of former Guantanamo Bay detainee Jihad Diyab, after he had resettled in neighboring Uruguay, saying he could now be in Brazil.

Last week, Brazil deported a French-Algerian nuclear physicist convicted by a Paris court in 2012 for his involvement in helping to plot an attack in France in 2009 with an al Qaeda militant in Algeria.

Brazil's intelligence agency said on Tuesday it was investigating all threats to the Rio Olympics, after a previously unknown Brazilian Islamist group calling itself "Ansar al-Khilafah Brazil" pledged allegiance to IS. It was unclear whether those detained on Thursday have any relation with that group.

On Wednesday, SITE Intelligence Group, a U.S. company that monitors the internet, said alleged Islamist militants were giving advice for lone actors to carry out attacks during the Games, including the use of toy drones. The messages are being sent via the mobile messaging system Telegram, SITE said.

Stratfor's Stewart said that, with no ability to project terrorist operatives into Brazil, IS could only seek to encourage grassroots groups. Brazil has one of the world's largest Arab diasporas, mostly Christians of Syrian and Lebanese origins.

After last week's attack in Nice, France, Brazilian authorities said they would step up security measures for the Games by adding further roadblocks and cordons and frisking more visitors in Rio de Janeiro. Brazilian police have traveled to Nice to study the attacks there, officials said.

(Additional reporting by Alonso Soto in Brasilia and Sergio Spagnuolo in Curitiba; Writing by Daniel Flynn and Paulo Prada; Editing by Meredith Mazzilli)