And tonight, we are all watching the raw chaos in a country america needs as a friend. Egypt with the world's largest arab population, so crucial to stability, democracy and america's fight against... See More
And tonight, we are all watching the raw chaos in a country america needs as a friend. Egypt with the world's largest arab population, so crucial to stability, democracy and america's fight against terror. And all day, these are the images that have bee pouring in. A woman, protecting an injured man by defiantly blocking an army bulldozer. A police vehicle being pushed from a bridge, falling 50 feet to the ground. And we could hear the sounds of fire power and fear. that is what you heard throughout the city of cairo today. Thousands running for their lives, as abc's muhammad lila, who knows this region so well, was right there out on the streets. Reporter: The assault was vision vicious. Tear gas scattering crowds including women and young children. Then came the bulldozers, raging fires and finally this -- heavy armed police opening fire. A steady stream of casualties flooding into makeshift hospitals. This footage shows moments of terror. A sniper on a rooftop calmly takes aim, firing into the crowd. Each time the violence appeared under control. It flared up again. As we're walking closer to the scene of the violence, we can feel the thickness, the tear gas in the air. And you can hear gun shots still running out. Ordinary egypt chans caught in thoss fire. One young woman showed her the view from her window. Chaos, bloodshed below. She was too afraid to speak to us. We spoke with a father and daughter who raced home from work early, their smokey seet now a ghost town. It's possible to attack anyone, because the streets are empty. So, it's still not safe but it's safe inside the house. Reporter: For u.S. Officials, it's a nightmare scenario. The key u.S. Ally overthrowing its elected leader, a hard-line member of the muslim brotherhood. Its supporters now facing off against the hardline military. Protesters fought back with rocks, even pushing this armored vehicle off a bridge. Crashing down, a symbol of the country's dissent into violence. Violence that is spreading. Angry mobs torching at least 18 churches in retaliation. As night falls, an eerie silence on the streets. No one knowing what horrors tomorrow will bring. Muhammad lila, abc news, in cairo. And so now, let's go to abc's chief white house correspondent, jonathan karl. 1 billion of military aide to egypt. What does the u.S. Do now? Reporter: Well, we heard some harsh words from the top of the obama administration today. John kerry called it deplorable. They repeatedly warned egyptians not to crack down on this and they ignored it. We just heard words, diane. There is no suggestion that anything will be done to punish egypt for this. But the flow of $1 billion in aide wi aid will continue to flow. And we keep seeing, there's so much resting on this, tell us the first and maybe most important thing, most important reverberation we'll feel in the u.S. Reporter: Just think of the terror threat. The fear here is that chaos in egypt will spreadghout that region. This is the most important, biggest country in the arab world, by far. The fear is that chaos breeds extremism, which breeds terrorism and you saw what happened, we had a global terror threat that shut down all of our embassies. They cannot see something like this spread further from egypt. Crucial days ahead. Thank you, jonathan karl.
This transcript has been automatically generated and may not be 100% accurate.