The protesters were so numerous that they blocked entire freeways, according to an AFP report, which says they chanted, "One point, One demand: Atheists must be hanged."
A video, above, shows raw footage from the fighting. Some of the protesters have apparently set car tires on fire. At one point, riot police can be seen firing into the crowds, presumably with rubber bullets. Another shot shows an entire street in flames.
Police gave varying figures for the size of the demonstration, though some estimated as many as 100,000 or even 200,000. Even if that's inflated, it's still a sign of the apparent force behind this movement. A number of the protesters reportedly came from outside the capital for the demonstration, part of a rising movement demanding harsher punishment for "insults" to Islam.
"This government does not have faith in Allah," one protester told the AFP, referencing Bangladesh's officially secular constitution, adopted in 1972. "This is an atheist government, we will not allow them to live in Bangladesh. Muslims are brothers, we must protect Islam."
The violence seems to have begun when police approached some stick-wielding protesters; it's not clear who began the fighting, but it escalated dramatically.
Bangladeshi Prime Minister Sheikh Hasina has resisted demands for tougher anti-blasphemy legislation, both because of the government's secular constitution and because, as she points out, the country already has such laws. She pointed out, as a way of calming the dissent, that the government has already arrested four bloggers for "derogatory comments" about Islam. That probably has not done much to calm Bangladesh's Hindu minority or other non-Muslims in the country.