COPENHAGEN, December 29, 2010 (AFP) - Scandinavian intelligence agencies on Wednesday said they had foiled a "Mumbai-style" plot by Islamic extremists to massacre staff at a Danish newspaper which published caricatures of the Prophet Mohammed.

Denmark's PET intelligence service said five suspects had been arrested, preventing an imminent assault on the Copenhagen offices of the Jyllands-Posten daily in which as many staff as possible would have been killed.

Four men were arrested in Denmark while a spokeswoman for Swedish intelligence agency Saepo said a fifth was arrested in Sweden in connection with the same international plot.

"It is our sense based on intelligence that this is a militant Islamic group with links to international terrorist networks," PET head Jakob Scharf told reporters.

They were planning an attack "within the next few days", the agency said in a statement.

In an email to Danish news agency Ritzau, Danish Justice Minister Lars Barfoed said the arrests prevented what could have been the most serious attack to ever occur in Denmark.

Scharf told the news conference "the plan was to try to gain access to the location of Jyllands-Posten in Copenhagen and to try to carry out a Mumbai-style attack".

The 2008 attacks in Mumbai saw 10 heavily armed gunmen storm three luxury hotels, the city's main railway station, a popular tourist restaurant and a Jewish centre. The attacks, centered around the luxury Taj Mahal Palace hotel, left 166 people dead.

"These arrests have successfully stopped an imminent terror attack, where several of the suspects... were going to force their way into the (building which houses Jyllands-Posten) in Copenhagen and kill as many people as possible," Scharf was quoted as saying in the PET statement.

Wednesday's arrests took place after a long investigation led in collaboration with Sweden's Saepo, PET said.

The man arrested in Stockholm is a 37-year-old Swede of Tunisian background.

Danish intelligence said the four men arrested in Denmark, in the Herlev and Greve suburbs of Copenhagen, were a 44-year-old Tunisian, a 29-year-old Swede born in Lebanon, a 30-year-old Swede and a 26-year-old Iraqi asylum seeker.

The first three were all living in Sweden and travelled to Denmark overnight. According to Jyllands-Posten's online edition, they travelled in a car rented in the Stockholm suburb of Kista.

Also at the Copenhagen press conference, Saepo head Anders Danielsson said the men based in Sweden had been under surveillance.

He added Saepo knew there were weapons in the car used for the trip to Copenhagen.

Jyllands-Posten published a dozen cartoons in 2005 of the Prophet Mohammed that triggered violent and sometimes deadly protests around the world.

"The arrests underscore the serious terror threat against Denmark and especially against institutions and people connected to the cartoon case," Scharf said Wednesday.

Lars Munch, the chief executive of Jyllands-Posten's parent company, said the company had "great confidence in the police and intelligence service efforts to protect us".

"It is shocking for our employees and their families to once again see their place of work threatened," he said in a statement.

PET said police seized "plastic strips that could have been used as handcuffs, a sub-machine gun with silencer as well as ammunition" in connection with the arrests.

An Islamist militant who killed only himself in Sweden's first suicide attack on December 11 said in a message sent before his death he wanted to punish Swedes for their "support of the pig Lars Vilks", a Swedish cartoonist who drew an image of Mohammed with the body of a dog in another publication.

Saepo spokeswoman Katarina Sevcik said the group arrested on Wednesday "have up to now no known connection to the events of December 11".

The PET said last month that there were "renewed indications that terrorist groups abroad are looking to send terrorists to Denmark to commit terrorist attacks".