The health ministry, which operates under the Hamas authority, has said Israeli attacks have killed at least 509 people in Gaza since the ceasefire came into effect.
The 74-year-old branded it "an attack on free speech" in a statement the foundation sent to media outlets, which noted he had been taken to a south London police station.
The Kremlin, which has bombarded Ukraine's energy grid for weeks, said on Friday it was refraining from attacks on Kyiv until Sunday following an appeal from US President Donald Trump.