(CNN) -- A Swedish SWAT team backed by local police arrested four people overnight on suspicion of plotting terror attacks, the Swedish Security Service told CNN Sunday.

The four were held in the city of Gothenburg for "probable cause" of preparing the attacks, the highest level of suspicion in Sweden, said Sara Kvarnstrom, a spokeswoman for the security force.

She refused to say whether the suspects had been under surveillance or if the arrests resulted from a tip-off, saying she could not discussed details on an ongoing investigation.

"Through these arrests we have been able to prevent a situation from occurring," Malena Rembe, head analyst at the Swedish Security Service, told CNN affiliate TV4.

Sweden is not raising its terror threat level -- which is currently at 3, with 5 as the highest -- and there is "no reason for the public to be alarmed," she said.

"These arrests have not changed this threat level," Kvarnstrom added.

An art gallery in central Gothenburg was evacuated shortly before midnight local time, police said, but declined to say whether it was connected to the arrests.

A party was going on at the Roda Sten gallery at the time, with about 500 people attending the inauguration of an arts festival, according an interview with a witness in Goteborgs Posten, a local newspaper.

The police cordon was removed at 6 a.m. local time, said Ulf Edberg, a spokesman for Gothenburg police.

Police have a "number of extra police officers" on the streets of Gothenburg, Edberg said.

What are the answers to this following set of questions:

1. How many people were arrested?
2. By who?
3. Who helped them?
4. Why the arrest?
5. Who spoke for the police?
6. Who was the spokesperson for them?
7. What is the concern threshold in Sweden?
8. Should citizens be worried?
9. What location was cleared of citizens?
10. When?
11. Why were the citizens there?
1. four
2. Swedish SWAT
3. local police
4. suspicion of plotting terror attacks
5. Swedish Security Service
6. Sara Kvarnstrom
7. currently at 3
8. no
9. An art gallery
10. shortly before midnight
11. For a party