Dubai, Jan. 9 -- Iran's supreme leader signalled Friday that security forces would crack down on protesters after they screamed from windows and marched through the streets overnight, directly challenging US President Donald Trump's pledge to support those peacefully demonstrating.

Supreme Leader Ayatollah Ali Khamenei dismissed Trump as having hands "stained with the blood of Iranians" as supporters shouted "Death to America!" in footage aired by Iranian state television.

Protesters are "ruining their own streets to make the president of another country happy," Khamenei said, referring to Trump. There was no immediate response from Washington, though Trump reiterated his pledge to strike Iran if protesters are killed.

Despite Iran's t...