I think this is the largest rally I've ever seen. The screen shot is a small section of it. Wait until the camera pans.
Yemen is experiencing thousands of bombing deaths and even more by massive starvation because it's being systematically destroyed by the US, Saudi Arabia and several of the gulf state monarchies, with almost no media notice until it was Donald Trump in the White House instead of Barak Obama and even now there's not much news about it in mainstream media.
The basic issue is power over the Arabian peninsula, particularly Sunni power. Yemen has always been kept impoverished and weak, but there was a rebellion in Yemen following the Arab Spring and a group called the Houthis took over. They are Shia and have the backing of Iran, which Saudi Arabia would also like to destroy because it's Shia and it threatens their Middle East supremacy.
The US backs Saudi Arabia because it uses its tremendous wealth to buy US arms and treasury notes and it's a reliable conservative ally for reliably conservative US foreign policy no matter which party is office.