Thousands of high-level federal employees turned to the court system Thursday to try to block requirements that would for the first time publish their personal financial information online.

The American Civil Liberties Union, in a lawsuit filed at the federal courthouse in Greenbelt, called the Internet disclosure provision an “unprecedented invasion of privacy” that would put nearly 28,000 government workers and their families at high risk for identity theft and other financial fraud.

“This publication will not be made by some hacker or leaker, but by the federal agencies that employ these individuals,” said the complaint filed by the ACLU of the Nation’s Capital and attorney Jack McKay on behalf of four employee organizations and seven individual workers.

At issue is a requirement, scheduled to take effect Sept. 30, that was initially designed to address allegations of insider trading by members of Congress and their aides. The law was expanded, without a public hearing, to require diplomats, scientists, immigration judges and other executive branch employees to disclose, online, assets such as real estate investments, retirement funds and stocks holdings.

The so-called Stop Trading on Congressional Knowledge Act has led to an outcry from federal workers and a bipartisan group of former national security and law enforcement officials.

As recently as Thursday afternoon, House and Senate lawmakers responded to concerns about financial privacy and national security. Before Congress adjourned, lawmakers delayed the online posting provision — which had been set for Aug. 31 — by a month.

Opponents of the disclosure provision argue that releasing such information would be a “jackpot for enemies of the United States intent on finding security vulnerabilities they can exploit,” according to a letter to Congress signed by former senior officials, including former Homeland Security secretary Michael Chertoff, former CIA director Michael Hayden and former Attorney General Michael Mukasey.

For decades, executive branch employees have been required to file disclosure forms about their investments and liabilities. Individuals trying to access those records have had to submit a formal request giving a valid reason for seeking the information.

Under the new provision, federal agencies have to create databases to give the public unrestricted access to search, sort and download such financial disclosure information.

In seeking a preliminary injunction from the federal court, attorneys for the employees argued that the online requirement violated the employees’ constitutional right to “informational privacy.”

Even though the government already collects financial information from executive branch employees, the attorneys say it must “show a compelling interest that the information be disclosed and that it has chosen the least intrusive method consistent with fulfilling that interest. It can do neither.”

On a more practical level, the suit quotes one of the plaintiffs, Joshua Zimmerberg, who says that the disclosure requirements will hurt efforts to recruit “capable scientists to public service.”

“I am already aware of individuals that have resigned positions or refused to apply” because of the obligation, said Zimmerberg, a senior scientist at the National Institutes of Health. He said in an interview that he is already prohibited by ethics regulations from investing in any company related to his work.

The lawsuit names as a defendant the president’s Office of Government Ethics, which is charged with posting the information on its Web site. President Obama’s choice to lead the office, Walter Shaub, said at his confirmation hearing last month that the law should be “changed to provide additional protections” for employees, striking a balance between privacy and helping the public “understand the financial motivations and interests of its leaders.”