The protest group Prisoners for Palestine on Tuesday shared a video showing the 22-year-old Swede holding a sign supporting the hunger strikers and their organisation, known as Palestine Action.
The British government earlier this year banned Palestine Action as a terrorist organisation.
The protests were part of a larger demonstration in which two other activists sprayed red paint in front of an insurance company in London, in an area known as the hub of Britain's financial services industry.
Prisoners for Palestine says they targeted the insurer because it supports the Israel-linked defence firm Elbit Systems.
City of London police said a man and a woman have been arrested on suspicion of criminal damage.
A third woman was later arrested on suspicion of supporting a banned organisation.
British police generally do not identify suspects by their names prior to their being charged.
Eight Palestine Action members have staged a hunger strike to protest their detention without bail as they await trial on a variety of charges related to earlier protests around the country.
The first two prisoners to join the protest have now been on hunger strike for 52 days and are at a "critical stage, where death is a real possibility," Prisoners for Palestine said in a statement.
The British government has so far refused to intervene in the judicial process, saying questions about bail and detention are matters for the courts to decide.
