German prosecutors on Thursday said four Hamas members had been arrested for planning attacks on Jewish institutions in Europe.
Separately, Denmark said it had detained three terrorism suspects, while another was arrested in the Netherlands.
Israel later claimed the suspects were Hamas members, but Denmark has not confirmed the claims.
“Following the terrible attacks by Hamas on the Israeli population, attacks on Jews in Jewish institutions have also increased in our country in recent weeks,” said German Justice Minister Marco Buschmann in a statement on the detentions.
“We must therefore do everything we can to ensure that Jews in our country do not have to fear for their safety again,” he said.
“The protection of Jews is our top priority,” Nancy Faeser, Germany’s Interior Minister, said on Thursday night.
The four people arrested in Germany are believed to be members of Hamas, with prosecutors saying they were under orders to bring a hoard of weapons from an undisclosed location in Europe to Berlin to attack Jewish institutions.
As part of the investigation, police searched five apartments and a restaurant in Berlin.
Officials said the people were planning attacks across Europe.
All the arrests were linked to a single, cross-border European terror plot, Israeli officials said.
Dutch citizen Nazih R was arrested by police in Rotterdam, while Lebanon-born Abdelhamid Al A and Ibrahim El-R, as well as Egyptian Mohamed B, were arrested in the German capital, prosecutors said. Germany does not reveal full identities of suspects.
All four have been long-standing members of Hamas with close links to the leadership of the group's military branch, they said.
Abdelhamid Al A had been tasked by Hamas leaders in Lebanon with sourcing weapons. The weapons were due to be taken to Berlin and kept at the ready for potential terrorist attacks against Jewish institutions, prosecutors said.
Dutch police said a 57-year-old man was arrested in Rotterdam on Thursday on request of German authorities in a Danish-German investigation.
Separately, Danish police said three people had been arrested in Denmark and one in the Netherlands on suspicion of preparing a terrorist attack but did not give further details on the suspects' motives.
In Brussels, Danish Prime Minister Mette Frederiksen said the arrests were “extremely serious”.
“It is of course – in relation to Israel and Gaza – completely unacceptable for someone to bring a conflict elsewhere in the world into Danish society,” she told news agency Ritzau at a meeting with EU leaders.
“For a number of years now, we have seen that there are people living in Denmark who do not wish us well. Who are against our freedom and who are against Danish society, with all that it entails,” Ms Frederiksen said.
The Danish Security and Intelligence Service (PET) said in August that the threat against Denmark had intensified after anti-Islam activists damaged several copies of the Quran over the summer.
European authorities have warned of an increased risk of attacks by Islamists radicalised by the Israel-Hamas war.
The European Union last week earmarked €30 million to protect places of worship amid heightened fears of terrorist attacks during the Christmas holiday season.
“The investigation has revealed that a network of people has been preparing a terrorist act,” Flemming Drejer, PET chief superintendent, said in Copenhagen.
“The arrests and the raids we're carrying out today are based on an intensive investigation that PET has carried out in close co-operation with our partners abroad,” he said.
Danish police said raids were ongoing across the country and were carried out at an early stage of the investigation.
The network had links to organised crime in Denmark and abroad, including to a gang named Loyal to Familia, or LTF, Mr Dreyer said, but declined to elaborate on possible motives.
Israel also claimed that Denmark had arrested seven people who, operating “on behalf of” Hamas, were planning an attack on civilians.
Israel's Mossad spy agency said that the Danish agencies had exposed “Hamas infrastructure on European soil”, according to a statement from Prime Minister Benjamin Netanyahu's office.
Danish police said they would increase their public presence in coming days, in particular in Copenhagen and around Jewish localities.
The Jewish community in Denmark said it had been briefed before the public about the raids but did not have any knowledge about concrete threats to Jewish targets.
PET has for more than a decade warned against potential attacks. Police said they had kept unchanged their terrorist alert level at four on a scale of one to five, reflecting a significant threat.
The three arrested in Denmark will be charged under the terrorism clause of the criminal code and face a judge for preliminary questioning, police said.
The three people arrested in Denmark are due to appear at a closed court session on Friday.
There is not thought to be a British connection to the Hamas terrorism plot, but the Community Security Trust, the body that provides security advice and protection to Jewish sites, said the news had worrying implications for the UK.
“Historically, Hamas has never shown interest in carrying out terrorist attacks outside of Israel,” a spokesman said.
“There have been one or two plots over the years linked to them but nothing on a significant scale.
“But if this is a shift in policy for Hamas to carry out attacks on Jewish communities outside the region, in line with Iran and Hezbollah, that would be extremely concerning.
“It represents a significant shift in the threat posed to Jewish communities.
“There is a big concern if Hamas HQ is ordering Hamas people in Europe to carry out an attack.”
