A new era of a la carte television arrived in earnest this week — seemingly all at once and more quickly than many industry executives and television fans had expected. And with it, the virtual monopoly that first, broadcast, and later, cable, satellite and telecommunications companies have had over TV programming is dissipating.

One day after HBO said it would start an Internet-only offering, CBS announced Thursday its own subscription Internet streaming service, which lets people watch its live programming and thousands of current and past shows on demand.

The moves signal a watershed moment for Web-delivered television.

The model allows viewers more options to pay only for the networks or programs they want to watch — and to decide how, when and where to watch them. Rapidly fading are the days in which people pay an average of $90 a month for a bundle of networks from a traditional provider.

“Everybody is talking about it,” Leslie Moonves, chief executive of CBS Corp., said in an interview. “It is an important part of our future. Our job is to do the best content we can and let people enjoy it in whatever way they want. The world is heading in that direction.”

Moonves wants his channel to stay relevant to a new generation of “cord nevers,” who have never paid for a standard television package, and “cord cutters,” who have canceled their cable television service.

The push into Web-only offerings by HBO and CBS, two networks that earn billions of dollars in profits from the traditional system, highlights how rapidly the balance of power is shifting in the television landscape. The “CBS All Access” service, at $6 a month, had its debut Thursday; details about HBO’s service, which is to start in 2015, are hazy.

The initiatives are largely a reaction to the success of Netflix, whose streaming service has more than 50 million global subscribers.

Along with Netflix, other insurgents such as Amazon and Hulu now offer on-demand programming, which can be watched anytime and anywhere on a laptop or a smartphone. New technologies, including “smart” televisions and streaming devices such as Roku and AppleTV, also allow viewers to watch Internet-delivered video on a big screen.

Reed Hastings, the chief executive of Netflix, said in an interview that the new wave of streaming options from traditional outlets validated his company’s long-held belief that the Internet was replacing traditional television, apps were replacing channels, remote controls were disappearing, and screens were proliferating. He said increased competition would force Netflix to work harder but would evangelize Internet video.

“We are Internet disrupters, through and through,” he said. “We are continuing to push that edge.”

Media executives are eager to appeal to the fast-growing number of viewers who pay for Internet but watch TV via cheaper streaming alternatives or free video options such as YouTube and other social media. The growth of subscriptions to cable and satellite services has stalled, dropping 0.5 percent to 101.4 million this year from 101.9 million in 2012, according to SNL Kagan. Among people ages 18 to 34, about one in six said they did not watch any original television series on a traditional television set in the past 30 days, according to comScore.

Sports are one major component holding the cable bundle together. Notably, National Football League games will not be available for the new CBS service. Executives at the network said they are in discussions with the NFL and that other live sports, such as, NCAA basketball’s March Madness are already available for streaming.

Many viewers will continue to pay for cable or satellite because of live sports programming on ESPN and other networks, which pay sports leagues big dollars for the rights to broadcast games. Live matches are one of the few remaining types of programming that still draw high ratings, and analysts say that sports fans who want to watch them will not be able to cut the cord. That said, a growing number of sports options exist on the Web with a traditional pay-TV package, including ESPN3. And some sports leagues, such as Major League Baseball, sell Internet-only subscriptions to their games.

The networks that offer new streaming services must perform a delicate balancing act so as not to cannibalize the billions of dollars that cable and satellite operators pay them to distribute their programming. CBS and HBO were careful to say that they would work with current and new business partners and that their new initiatives would not cut into their existing business.

Some analysts were skeptical. The new stand-alone Web services could cause cable companies to demand that CBS and HBO charge lower fees for their programming. To hold on to potential cord cutters, cable companies such as Comcast and Charter Communications could be forced to create more segmented packages. But in some ways, the new Web-only products could benefit the cable companies, which make billions of dollars selling Internet service into the home. Netflix, and now CBS and HBO, need broadband service to reach customers over the web.

CBS and HBO are not the only two new Web-only offerings. Sony is preparing an Internet product expected to include programming from Viacom, the parent of networks Comedy Central, MTV and Nickelodeon. DirecTV also said that it would start an online video service. A similar service from Showtime, the premium cable network owned by CBS, is likely in the “not too distant future,” Moonves said.

It remains to be seen how many different Web-TV subscriptions viewers will pay for, especially if the total adds up to more than the cost of a traditional cable package. Netflix starts at $8 a month, and Hulu’s premium service costs $8 a month. An Amazon Prime membership, which includes video streaming, is $99 a year. But one thing is certain: Viewers today have more power to create their own television bundles.