Facebook CEO Mark Zuckerberg began a two-day congressional inquisition Tuesday with a public apology for a privacy scandal that has roiled the social media giant he founded more than a decade ago.

Zuckerberg opened his remarks before the Senate Commerce and Judiciary committees by taking responsibility for failing to prevent Cambridge Analytica, a data-mining firm affiliated with Donald Trump's presidential campaign, from gathering personal information from 87 million users to try to influence elections.

Zuckerberg had apologized many times already, to users and the public, but this was the first time in his career that he had gone before Congress. He also is to testify Wednesday before the House Energy and Commerce Committee.

In the hearings, Zuckerberg is not only trying to restore public trust in his company but also to stave off federal regulations that some lawmakers have floated. In his opening statement to senators, he also apologized for fake news, hate speech, a lack of data privacy and Russian social media interference in the 2016 elections.

"We didn't take a broad enough view of our responsibility, and that was a big mistake," he said. "It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here."

Separately, the company also began alerting users that their data was gathered by Cambridge Analytica. A notification that appeared on Facebook for some users Tuesday told them that "one of your friends" used Facebook to log into a now-banned personality quiz app called "This Is Your Digital Life." The notice says the app misused the information, including public profiles, page likes, birthdays and current cities, by sharing it with Cambridge Analytica.

After resisting previous calls to testify, Zuckerberg agreed to come to Capitol Hill this month after reports surfaced — and the company confirmed — that Cambridge Analytica had gathered Facebook users' data. Zuckerberg said his company has a responsibility to make sure what happened with Cambridge Analytica doesn't happen again.

Zuckerberg is also expected to be asked about Russia's use of U.S. social media during the 2016 elections — a subject of several congressional investigations and special counsel Robert Mueller's probe into Russian interference.

In his opening statement, Zuckerberg addressed Russian election interference and acknowledged, as he has in the past, that the company was too slow to respond and that it's "working hard to get better." The company has said that as many as 146 million people may have received information from a Russian agency that's accused of orchestrating much of the cyber meddling in the election.

"We will continue working with the government to understand the full extent of Russian interference, and we will do our part not only to ensure the integrity of free and fair elections around the world, but also to give everyone a voice and to be a force for good in democracy everywhere," Zuckerberg continued.

In the testimony, Zuckerberg acknowledged that the questioning could be hostile.

"We face a number of important issues around privacy, safety, and democracy, and you will rightfully have some hard questions for me to answer," Zuckerberg said.

His opening statement does not reveal new information about how data was shared or what Facebook will do. In addition to saying he is sorry, Zuckerberg outlined the steps the company has taken to restrict outsiders' access to people's personal information. He also said the company is investigating every app that had access to a large amount of information before the company moved to prevent such access in 2014 — actions that came too late in the Cambridge Analytica case.

Zuckerberg met Monday with Florida Sen. Bill Nelson, the top Democrat on the Senate Commerce panel, along with other senators. Nelson said afterward that Zuckerberg was "forthright and honest to the degree he could" be in the private, one-on-one meeting.

Nelson said he believes Zuckerberg is taking the congressional hearings seriously "because he knows there is going to be a hard look at regulation."

Democrats like Nelson have argued that federal laws might be necessary to ensure user privacy. Republicans have yet to get behind any such legislation, but that could change.

Texas Sen. John Cornyn, a member of the Judiciary panel and the No. 2 Republican in the Senate, appeared open to regulation in a Tuesday floor speech ahead of the hearing. Cornyn said apologies are "not enough" and suggested that legislation could eventually be needed to give consumers more control over their own data privacy.

"This is a serious matter, and I think people expect us to take action," Cornyn told reporters after his speech.