The new boss of Twitter, Elon Musk, suggested Friday that “activist groups” are pressuring advertisers to walk away from the social media platform and that their efforts have led to what he described as a “massive drop in revenue.”

In a tweet posted to the platform, Mr. Musk said that the pressure campaign was launched even though nothing has changed regarding Twitter’s content moderation efforts and that he and his team did “everything we could to appease the activists.”

He added that what the activists are doing is “extremely messed up,” saying that they are “trying to destroy free speech in America.”

In the days since Mr. Musk took over Twitter, a number of companies have reportedly paused advertising on the platform while the dust he kicked up in the process of buying it settles.

The Wall Street Journal said major players such as General Mills, Mondelez International, Pfizer, and Volkswagen are worried that Mr. Musk’s planned retreat on moderation could lead to an increase in objectionable content; others are concerned about the exodus of executives and want to see what changes he makes before restarting the ads.

The Journal cited one advertising industry executive as saying that about 20 of its clients had suspended advertising. General Motors said last week that it was doing the same, a move it described as normal when a media platform undergoes substantial change. “We are engaging with Twitter to understand the direction of the platform,” GM said.

The same day GM announced its intentions, Mr. Musk sought to reassure advertisers in an open letter posted on the platform. “There has been much speculation about why I bought Twitter and what I think of advertising,” he said. “Most of it has been wrong.”

Mr. Musk stated his vision of Twitter as a “digital town square” where “a wide range of beliefs can be debated in a healthy manner.” Many media companies, he said, have catered to polarized extremes in society in order to enhance their revenues. He hinted that his motivation for buying the platform is to prevent Twitter from continuing down that same path.

“Twitter obviously cannot become a free-for-all hellscape, where anything can be said with no consequences,” he said. The platform, he said, would remain “warm and welcoming to all.”

After posting the letter, Mr. Musk reportedly has taken several private meetings with major advertisers in an effort to make similar reassurances. In those meetings, according to the Journal, Mr. Musk stressed that the platform would remain a safe space for brands and that he hoped to segment off content on the platform and let users and brands decide which of those sandboxes they want to play in — something akin to cinema’s G-, PG- and R-ratings.

Twitter relies on advertising for the bulk of its revenue, so any slowdown in spending by the big advertisers could affect the company. In the second quarter of this year, Twitter generated $1.18 billion in revenue, $1.08 billion of which came from advertising.

In an effort to cut costs at a company now saddled with considerable debt after Mr. Musk’s $44 billion takeover, Twitter announced Friday that significant layoffs were in the works across the company. It is unclear how many of the platform’s 7,500 employees would be let go, but some reports have suggested that as many as half could be handed walking papers.