Kabul falls to the Taliban as the Afghan government collapses and the president flees.

The Taliban effectively sealed their control of Afghanistan on Sunday, pouring into the capital, Kabul, and meeting little resistance as President Ashraf Ghani fled the country, the government collapsed, and chaos and fear gripped the city, with tens of thousands of people trying to escape.

The insurgents’ return to power, two decades after they were ousted, came despite years and hundreds of billions of dollars spent by the United States to build up the Afghan government and its defense forces. In a lightning offensive, the Taliban swallowed dozens of cities in a matter of days, leaving Kabul as the last major redoubt of government control.

On Sunday evening, former President Hamid Karzai announced on Twitter that he was forming a coordinating council together with Abdullah Abdullah, chairman of the Afghan delegation to peace talks, and Gulbuddin Hekmatyar, the leader of the Hesb-i-Islami party, to manage a peaceful transfer of power. Mr. Karzai called on both government and Taliban forces to act with restraint.

But the Taliban appeared to ignore his appeal and advanced into the city on its own terms.

The Taliban’s lead negotiator in talks with the government, Mullah Abdul Ghani Baradar, congratulated all of Afghanistan for the victory. “Now it will be shown how we can serve our nation,” he said. “We can assure that our nation has a peaceful life and a better future.”

Mr. Baradar made the comments in a video posted on social media, surrounded by other members of the Taliban delegation to the talks in Doha, Qatar.

“There was no expectation that we would achieve victory in this war,” he said. “But this came with the help of Allah, therefore we should be thankful to Him, be humble in front of Him, so that we do not act arrogantly.”