Pakistani authorities said the earthquakes have killed at least 500 people in that country's North West Frontier Province, which borders Afghanistan.
Authorities said several Pakistani villages have been wiped out and reported early on that the eventual death toll could top 1,000.
One Terrifying Minute...
The initial quake lasted a terrifying minute. In the Pakistani capital Islamabad, people were heard screaming in fear inside their homes. Then, within minutes, the rescue operation began. Emergency services raced through the city, as residents desperately picked through rubble to reach those trapped under two collapsed apartment blocks.
Many sat outside their homes, worried about aftershocks or another earthquake.
One Islamabad man said he'd never seen anything like it: "This morning we felt extremely strong tremors, and all of us woke up and ran out. The shocks went on for a long time. We have never seen an earthquake like this before."
...And Continuing Threats
The quake and four powerful aftershocks were felt across the subcontinent, shaking buildings in cities across Pakistan, India, and Afghanistan.
The Pakistani Army's chief spokesman, Major General Shaukat Sultan, said the final toll could be well over 1,000. The army has been called in to help with rescue efforts in one part a northern district of the North West Frontier Province.
Officials are now speaking of entire villages being wiped out in Pakistan-controlled Kashmir, close to the epicenter.
An army spokesman said the quake triggered landslides there, burying villages and an untold number of people. Casualties, he warned, could be "massive."
Vijay Bakaya, the chief secretary of India's Jammu and Kashmir state, said in Srinagar that the death toll in Indian Kashmir stood at 174 with another 600 people injured.
"In Baramullah district, 113 people have died and, out of this, 104 have died in Uri alone -- and that's the area in which most of the houses have also collapsed," Bakaya said. "Nearly 80 percent of the houses in Uri have collapsed or developed major cracks."
"We were studying in the class and suddenly the ground began to shake," said one boy, Afzal, who was in school when the quake hit. "We fell on the ground and ran out of the building."
In the Afghan cities of Kabul and Mazar-e Sharif, thousands of people rushed out into the streets fearing their homes would collapse.
Two people were initially reported to have died in Afghanistan -- one of them an 11-year-old girl in Nangahar Province, crushed when a wall in her home collapsed.
"The first reports we have are that the earthquake was felt in several provinces neighboring Kabul," RFE/RL correspondent Breshna Nazari reported from the Afghan capital. "But the earthquake was much stronger in the northern provinces of the country like Badakshan, Takhar, and Parwan. The earthquake there was quite strong."
The area where the quake hit is known for its frequent seismic activity. Experts have long predicted an imminent major earthquake in the region.
Officials called today's quake the worst in recent times.
Strong tremors were felt hundreds of kilometers to the northwest, according to Tajik seismologists.
"This morning at 8:52 local time, an earthquake was felt on the territory of Tajikistan," said Tatyana Valiasko of the Tajik Seismological Center told RFE/RL's Tajik Service. "The epicenter was 530 kilometers southeast of Dushanbe on the territory of Pakistan. Preliminary estimates say it measured 6.7.... Tremors measured 2-3.0 in Khujand, 5.5 in Khorog, and 4.0 in Dushanbe."
(with additional news agency reports)