I may be wrong, but I strongly suspect this large crowd is objecting to Israel's response to the attack by Hamas rather than supporting Hamas directly. IOW, they have friends and relatives in Gaza who are getting bombed, therefore they are protesting.The problem is optics.
This happened:
In the morning of 7 October, as Palestinian militants in Gaza launched a barrage of over 5,000 rockets against Israeli cities and kibbutzim while some 3,000 Palestinian militants breached the Gaza–Israel barrier. Over 1,000 Israeli civilians were killed in more than a dozen massacres, including the Re'im music festival massacre. Eight Israeli military bases situated near the Gaza border were attacked. Over 200 civilians and Israeli soldiers were abducted to the Gaza Strip. At least 44 countries, mostly from the Western world, characterized the massacres of civilians as terrorism
And then this:
Aerial view of Nov. 4th National Protest for Palestine in Washington D.C. Activists say the crowd is 100,000 people and growing.
It sure does "look like" support for the attack on Israel to me.....and I'm sure a lot of other people too.......
I cannot imagine why any large group of people would support a militant, terrorist organization.
