The man, who is in his 40s, is being investigated for links to the PCPA, Israel's Foreign Ministry writes on X. He is one of two participants from the convoy who are now in Israel for questioning.
Middle East expert Anders Persson describes the arrest as "a potentially hot potato."
There is a risk that this person will be tried in Israel. It could be that they have some kind of evidence, I don't know about that, but it could also be that Israel wants to set a deterrent precedent.
Flotilla activists who have previously been arrested or taken into custody by Israel have generally been sent out of the country as quickly as possible.
They are given the chance to sign some kind of deportation order, agreeing to be deported. It is usually a short legal process where they are put on a plane back to Europe, sometimes on a night flight, says Anders Persson.
Spain and Brazil
Israel classifies the PCPA as a terrorist organization. According to the country's Defense Ministry, the group acts as Hamas' representative body outside the Gaza Strip.
The organization behind the convoy, Global Sumud Flotilla, states that the man has both Swedish and Spanish citizenship. The other man brought to Israel is from Brazil.
"The governments of Spain and Brazil strongly condemn the kidnapping of two of their citizens in international waters by the government of Israel," the Spanish Foreign Ministry wrote on X.
Spain has extremely poor relations with both Israel and the US at the moment because of Israel and Iran, says Persson.
The embassies of Sweden and Spain have been granted consular access to the man, who has been resident in Spain for a few years, writes Foreign Minister Maria Malmer Stenergard (M) in a comment to TT.
The Ministry of Foreign Affairs sends reinforcements
The minister says she does not want to complicate the work and cites consular confidentiality for not going into details, but continues:
"The Ministry of Foreign Affairs has sent specially trained reinforcement personnel to support the embassies in Athens and Tel Aviv in their consular work."
"In contact with my Israeli counterpart, I have expressed the importance of respecting the consular rights of Swedish citizens."
In connection with the operation on Thursday, the Israeli military stopped around 20 of the boats in the Gaza convoy near Crete in the Mediterranean Sea. Several of the activists - including several Swedes - were taken ashore in Greece.
