U.S. lawmakers and luminaries expressed alarm at the extensive efforts allegedly made by a ring of Russian operatives to influence the 2016 presidential election, as outlined in a surprise indictment by the U.S. special counsel.

The indictment announced by U.S. Special Counsel Robert Mueller on February 16 revealed scores of previously unknown details, disclosing that Russian operatives not only placed social media ads and posts that reached millions of U.S. voters during the election, but they actually visited 10 U.S. "swing" states where the race was particularly close and helped stage political rallies boosting Republican Donald Trump and denigrating his opponent, Democrat Hillary Clinton.

Republicans and Democrats alike expressed disgust and alarm, with many stating their resolve to take action to prevent such foreign interference in U.S. elections again.

"We have known that Russians meddled in the election, but these indictments detail the extent of the subterfuge," said the speaker of the House of Representatives, Paul Ryan, calling it a "sinister and systematic attack on our political system."

"It was a conspiracy to subvert the process, and take aim at democracy itself," Ryan said. "We need to follow the facts and work to protect the integrity of future elections."

"[Russian President] Vladimir Putin has been using an arsenal of trolls, disinformation, and propaganda to try to undermine western democracies," said Representative Ed Royce, chairman of the House Foreign Affairs Committee.

"More must be done, including with international broadcasting, to counter this threat. Our midterm elections are rapidly approaching, and we must ensure the sanctity of our political process," he said.

A spokesman for Hillary Clinton -- who the Russian operatives actively sought to smear through media posts and rallies, according to the indictment -- said the charges confirmed what she has "long known."

"Russia went to great lengths to undermine our democracy," Nick Merrill said on Twitter. "No matter your politics, it's un-American. We have an adversary that is laughing at us, who will act again."

Senator Bernie Sanders, who vied with Clinton for the Democratic presidential nomination and whose campaign was also targeted by the Russian operatives, said: "It is the American people who should be deciding the political future of our country, not Mr. Putin and the Russian oligarchs. It is absolutely imperative that the Mueller investigation be allowed to go forward without obstruction from the Trump administration or Congress."

Former U.S. Attorney General Eric Holder called the indictment "hugely significant" on Twitter. "The contours of the conspiracy are starting to be detailed," he said.

Senator Mark Warner, the top Democrat on the U.S. Senate Intelligence Committee, said he would continue to press social media companies like Facebook and Twitter "to be far more aggressive and proactive in responding to this threat."

But he admitted that preventing all such activity on open media sites is likely to prove impossible and citizens of Western democracies must simply learn to be more cautious about what the read online.

"We each bear some responsibility for exercising good judgment and a healthy amount of skepticism when it comes to the things we read and share on social media," he said.

With reporting by AP and Reuters

