A former executive at ByteDance, the Chinese company that owns TikTok, has accused the technology giant of a “culture of lawlessness,” including stealing content from rival platforms Snapchat and Instagram in its early years, and called the company a “useful propaganda tool for the Chinese Communist Party.”

The claims were part of a wrongful dismissal suit filed on Friday by Yintao Yu, who was the head of engineering for ByteDance’s U.S. operations from August 2017 to November 2018. The complaint, filed in San Francisco Superior Court, says Mr. Yu was fired because he raised concerns about a “worldwide scheme” to steal and profit from other companies’ intellectual property.

Among the most striking claims in Mr. Yu’s lawsuit is that ByteDance’s offices in Beijing had a special unit of Chinese Communist Party members sometimes referred to as the Committee, which monitored the company’s apps, “guided how the company advanced core Communist values” and possessed a “death switch” that could turn off the Chinese apps entirely.

“The Committee maintained supreme access to all the company data, even data stored in the United States,” the complaint said.

Mr. Yu’s claims, which describe how ByteDance operated five years ago, are surfacing as TikTok faces intense national scrutiny over its relationship with its parent company and China’s potential influence on the platform. The video app, which is used by more than 150 million Americans, has become hugely popular for memes and entertainment. But lawmakers and U.S. officials are concerned that the app is passing sensitive information about Americans to Beijing.

In March, a congressional committee grilled TikTok’s chief executive, Shou Chew, about the app’s Chinese ownership. Christopher Wray, the director of the Federal Bureau of Investigation, recently said that TikTok “screams out with national security concerns.” More than two dozen states have banned TikTok from government devices since November.

In an emailed statement, ByteDance said Friday that the company would “vigorously oppose what we believe are baseless claims and allegations in this complaint.”

“Mr. Yu worked for ByteDance Inc. for less than a year and his employment ended in July 2018. During his brief time at the company, he worked on an app called Flipagram, which was discontinued years ago for business reasons,” the statement said.

In his complaint, Mr. Yu, 36, said that as TikTok sought to attract users in its early days, ByteDance engineers copied videos and posts from Snapchat and Instagram without permission and then posted them to the app. He also claimed that ByteDance “systematically created fabricated users” — essentially an army of bots — to boost engagement numbers, a practice that Mr. Yu said he flagged to his superiors.

Mr. Yu says he raised these concerns with Zhu Wenjia, who was in charge of the TikTok algorithm, but that Mr. Zhu was “dismissive” and remarked that it was “not a big deal.”

Mr. Yu, who spent part of his ByteDance tenure working in its China offices, said he also witnessed engineers for Douyin, the Chinese version of TikTok, tweak the algorithm to elevate content that expressed hatred for Japan. In an interview, he said that the promotion of anti-Japanese sentiments, which would make it more prominent for users, was done without hesitation.

“There was no debate,” he said. “They just did it.”

The lawsuit also accused ByteDance engineers working on Chinese apps of demoting content that expressed support for pro-democracy protests in Hong Kong, while making more prominent criticisms of the protests.

As an example of what was described as the “lawlessness” inside the company, the lawsuit says the founder of ByteDance, Zhang Yiming, facilitated bribes to Lu Wei, a senior government official charged with internet regulation. Chinese media at the time covered the trial of Lu Wei, who was charged in 2018 and subsequently convicted of bribery, but there was no mention of who had paid the bribes.

TikTok has sought to convince lawmakers that it operates at an arm’s length from ByteDance and that the Chinese government has no influence or special access to the app. It has been working on a costly plan to store American user data on servers operated by Oracle in the United States, known as Project Texas.

Mr. Yu, who was born and raised in China and now lives in San Francisco, said in the interview that during his time with the company, American user data on TikTok was stored in the United States. But engineers in China had access to it, he said.

The geographic location of servers is “irrelevant,” he said, because engineers could be a continent away but still have access. During his tenure at the company, he said, certain engineers had “backdoor” access to user data.

His lawsuit demands lost earnings, punitive damages and 220,000 ByteDance shares that had not vested by the time he was dismissed. The complaint does not cite a specific dollar amount in damages, but the shares alone would be worth tens of millions of dollars. The case was filed after several years of mediation with the company failed.

Mr. Yu is being represented by Charles Jung, a San Francisco lawyer who specializes in employment disputes.