Scotland Yard
The arrest is Syria-related, police said.
The 28-year-old said he was left fearing for his safety.
The party itself is not under investigation.
It relates to incidents at a number of hospitals, police say.
Another man was injured in a separate shooting.
The officers were responding to reports of a fight.
He was pronounced dead at the scene.
He described his “almost feral reaction” when he realised she was missing.
The 38-year-old was a tourist visiting the palace.
He was detained in west London.