The elder Mr. Katsyv is now a vice president of Russian Railways, a state-owned railroad monopoly that is the country’s largest employer and one long dogged by corruption allegations.
The junior Mr. Trump has said he accepted the meeting, which included two high-ranking aides of his father’s campaign, after an email from an associate had said that someone advertised as a “Russian government attorney” would deliver information compiled by the Russian state prosecutor that was damaging to Hillary Clinton.
It is not known exactly what Ms. Veselnitskaya said in the roughly 30-minute meeting. Donald Trump Jr. said she spent much of the time attacking American sanctions. Ms. Veselnitskaya has denied that, at the behest of Russian officials, she discussed compromising material with members of the Trump campaign team.
Dmitri S. Peskov, the spokesman for President Vladimir V. Putin of Russia, told reporters that the Kremlin had never heard of her. Asked about possible links between Ms. Veselnitskaya and the Foreign Ministry, Maria Zakharova, the ministry’s spokeswoman, said that it had “nothing to do” with her efforts.
But in Russia, lines between career, loyalty and government service tend to blur more than in other countries.
Ms. Veselnitskaya built her career in the sharp-elbowed struggle for land as the Moscow suburbs expanded, as once derelict factory sites and other plots became wildly valuable with the spread of shopping centers and new highways. And she forged an important connection to the Katsyv family.
William Browder, an American-born hedge fund manager who has tussled repeatedly with Ms. Veselnitskaya, said of the elder Mr. Katsyv: “In the world of Russia he’d be the equivalent of a Chris Christie: no formal relationship to the Kremlin, but with very strong relations to the powers that be.”
The family’s trust in Ms. Veselnitskaya was rewarded in May, when she helped Denis P. Katsyv, Pyotr’s son, fight the money laundering claims in New York brought by the Manhattan federal prosecutor at the time, Preet Bharara. Mr. Bharara tangled with Ms. Veselnitskaya several times and protested at one point that she had been charging the government for a $995-a-night room at the Plaza Hotel.
The case was settled two months after Mr. Bharara was dismissed by President Trump.
Prevezon Holdings, Mr. Katsyv’s company, paid $6 million to resolve the claim without admitting any crime. While the prosecution portrayed the settlement as a victory, Ms. Veselnitskaya told the newspaper Izvestia that it was “almost an apology from the government.”
In recent years, she had become the public face of Moscow’s efforts to reverse international travel and financial sanctions on key Russian figures linked to an alleged $230 million tax fraud.
Sergei Magnitsky, a lawyer who exposed the fraud, was arrested by the same prosecutors who he suggested had organized it. He died in jail in 2009 amid accusations of beatings and medical malpractice.
In 2012 Mr. Browder, who had been Mr. Magnitsky’s boss, successfully campaigned for the United States Congress to pass the Magnitsky Act, a collection of sanctions naming Russian officials linked to Mr. Magnitsky’s death. An outraged Mr. Putin responded by banning Americans from adopting Russian children.
Ms. Veselnitskaya has also met with members of Congress and helped to establish a Delaware nonprofit group that lobbied against the sanctions. She submitted lengthy testimony and organized a screening of an anti-Magnitsky film at Newseum in Washington in June 2016, just days after she met with Donald Trump Jr.
She also attended a congressional committee hearing on American policy toward Russia a day after the screening, taking a front-row seat.
“She has this kind of patriotic thing about her,” said Andrei Nekrasov, the Russian documentary maker who made the film, which critics called inaccurate. “She does have a tendency to go a bit solemn, if not pompous and say ‘My country is being attacked’ — that is her style.”
It was Ms. Veselnitskaya’s desire to get the United States to reverse the Magnitsky Act that prompted her to seek a meeting with the Trump campaign, she said Saturday in written responses to questions from The New York Times. On Tuesday, her office withdrew the promise of an interview.
Ms. Veselnitskaya started her career in the prosecutor’s office in Moscow’s suburbs before branching out. She earned a reputation as a fearsome opponent, intimidating both inside the courtroom and in the corridors, where she was known to threaten adversaries with the wrath of the government. By her own account in a recent United States legal filing, she said she had argued and won more than 300 cases.
One lawyer who has opposed her in court described her technique as 20 percent law, at which she excelled, and 80 percent acting. She would wave her hands and describe the travails of her victims in emotional terms, the lawyer said, who feared using his name for the safety of his family.
Mr. Putin’s government has long been criticized as favoring an elite of loyal and trustworthy insiders who are permitted to manipulate the courts and government agencies to promote their interests — so long as they toe the Kremlin line. Ms. Veselnitskaya, lawyers and others who have followed her career said, was well versed in this game and wielded its weapons in the mad dash for real estate profits in the Moscow region that often pitted factions of current and former regional officials against one another.
In one instance, a small nonprofit, Spravedlivost, that tried to expose corruption in the Moscow region, published a series of articles accusing a group that included Ms. Veselnitskaya, her former husband, Aleksandr Mitusov, and Mr. Katsyv of being corporate raiders who used their clout in the regional government and the courts to seize valuable land.
Ms. Veselnitskaya helped bring a defamation suit against the nonprofit and the factory owner who had accused her of grabbing property. The court hit the representatives of Spravedlivost and the factory owner with hefty fines.
In 2008, her work with her stepdaughter attracted the attention of Vladimir R. Soloviev, now a well-known talk show host on state-run television.
“They were involved in several very similar and very dubious stories,” Mr. Soloviev wrote in a much-shared blog post about the spread of corporate raiding in Russia. “Internecine strife, strange court decisions, documents that appear from nowhere and, as a rule, all connected with land plots in the Moscow region.”
In another such case, Ms. Veselnitskaya took on Ikea, claiming that some of the land under an office complex owned by the Swedish company on the outskirts of Moscow belonged to an old farming cooperative.
It was one of many cases brought against the Swedish giant, and a former board member said they never could figure out who was behind them. This case was ultimately dismissed by the Russian Supreme Court.
Emails from the younger Mr. Trump released Tuesday indicate that the meeting with the Trump campaign was organized through Emin Agalarov, the singer son of the billionaire developer Aras Agalarov, who was Mr. Trump’s partner in bringing the 2013 Miss Universe contest to Moscow.
Mr. Agalarov’s fortune is based partly on giant shopping centers built around Moscow by his Crocus Group, whose work would undoubtedly have brought him into contact with Mr. Katsyv — and Ms. Veselnitskaya.
Ms. Veselnitskaya seems to revel in her emergence as a player in Russian policy. On her Facebook page, decorated with the Russian flag, she has a penchant for articles about conspiracy theories, and has often handicapped political developments in the United States like the appointment of Neil M. Gorsuch to the Supreme Court or the dismissal of Sally Q. Yates as acting attorney general.
When a friend wrote that she wished that Yuri Chaika, the Russian state prosecutor, had the same principles as Ms. Yates, Ms. Veselnitskaya objected, accusing the Americans of being corrupt. “In the United States, politics has long been the most lucrative type of business,” she wrote.
Because of an editing error, a picture caption with an earlier version of this article referred incorrectly to a statement from President Vladimir V. Putin’s spokesman. The spokesman said Monday, not Tuesday, that Mr. Putin had never heard of Natalia Veselnitskaya.