(Adds details from announcement, Warner comment, background)
SAN FRANCISCO, Oct 24 (Reuters) - Twitter Inc said on Tuesday it would add labels to election-related advertisements and say who is behind each of them, after a threat of regulation from the United States over the lack of disclosure for political spending on social media.
Twitter said in a blog post the company would launch a website so that people could see the identities of the buyers, targeting demographics and total ad spend by election advertisers, as well as information about all ads currently running on Twitter, election-related or otherwise.
Silicon Valley social media firms and the political ads that run on their websites have generally been free of the disclaimers and other regulatory demands that U.S. authorities impose on television, radio and satellite services.
Calls for that to change have grown, however, after Twitter, Facebook Inc and Alphabet Inc's Google said in recent weeks that Russian operatives used fake names on their platforms to spread divisive messages in the run-up to the 2016 U.S. presidential election.
Russia has denied interfering in the election.
Twitter plans to make changes first in the United States and then roll them out globally.
Changes would appear within Twitter feeds, where election ads would have a label like "promoted by political account," the company said.
"To make it clear when you are seeing or engaging with an electioneering ad, we will now require that electioneering advertisers identify their campaigns as such," Bruce Falck, Twitter's general manager of revenue product, said in the blog post.
Twitter also said it would limit the targeting options for election ads, although it did not say how, and introduce stronger penalties for election advertisers who violate policies.
Democratic U.S. Senator Mark Warner said from his Twitter account that the moves by the company were a "good first step" but he added that Congress should make the disclosures mandatory by approving legislation he is co-sponsoring.
Separately, Twitter has long been criticized by users and lawmakers as lax in policing fake or abusive accounts. Unlike Facebook, Twitter allows anonymous accounts and automated accounts, or bots, making the service more difficult to police.
Twitter said last month it had suspended about 200 Russia-linked accounts as it investigated online efforts to influence last year's U.S. election.
The general counsels for Facebook, Google and Twitter are scheduled to testify next week before public hearings of the Senate and House intelligence committees. (Reporting by David Ingram in San Francisco; Additional reporting by Supantha Mukherjee in Bengaluru; Editing by James Dalgleish)