WASHINGTON — Internet activists declared victory over the nation’s big cable companies Thursday, after the Federal Communications Commission voted to impose the toughest rules yet on broadband service to prevent companies like Comcast, Verizon and AT&T from creating paid fast lanes and slowing or blocking web traffic.

The 3-2 vote ushered in a new era of government oversight for an industry that has seen relatively little. It represents the biggest regulatory shake-up to telecommunications providers in almost two decades.

The new rules require that any company providing a broadband connection to your home or phone must act in the “public interest” and refrain from using “unjust or unreasonable” business practices. The goal is to prevent providers from striking deals with content providers like Google, Netflix or Twitter to move their data faster.

“Today is a red-letter day for Internet freedom,” said FCC Chairman Tom Wheeler, whose remarks at Thursday’s meeting frequently prompted applause by Internet activists in the audience.

President Barack Obama, who had come out in favor of net neutrality in the fall, portrayed the decision as a victory for democracy in the digital age. In an online letter, he thanked the millions who wrote to the FCC and spoke out on social media in support of the change.

“Today’s FCC decision will protect innovation and create a level playing field for the next generation of entrepreneurs — and it wouldn’t have happened without Americans like you,” he wrote.

Verizon saw it differently, using the Twitter hashtag #ThrowbackThursday to draw attention to the FCC’s reliance on 1934 legislation to regulate the Internet. Likewise, AT&T suggested the FCC had damaged its reputation as an independent federal regulator by embracing such a liberal policy.

“Does anyone really think Washington needs yet another partisan fight? Particularly a fight around theInternet, one of the greatest engines of economic growth, investment and innovation in history?” said Jim Cicconi, AT&T’s senior executive vice president for external and legislative affairs.