More than 30 lawsuits have been filed in the United States against Equifax after the credit reporting company said thieves may have stolen personal information for 143 million Americans in one of the largest hackings ever.

At least 25 lawsuits had been filed in federal courts by Sunday, including at least one accusing the company of securities fraud, court records show.

Several more lawsuits were filed against Equifax on Monday. Many of those raising similar claims will likely be combined into a single, nationwide case.

​Equifax disclosed the breach on Thursday, and said it learned of the hacking on 29 July. It has set up procedures that it says are intended to help people protect their Social Security numbers and other identifying information.

Some lawsuits criticised Equifax’s offer of a year of free credit monitoring with its TrustedID product.

One complaint filed in San Jose, California, suggested that Equifax might do this to lay a “foundation” to pitch costlier services. It cited a 22 February regulatory filing in which Atlanta-based Equifax said more companies are offering free or low-cost services such as credit scores, reports and monitoring “as a means to introduce consumers to premium products and services.”

In the securities fraud lawsuit, Equifax was accused of misleading shareholders about its ability to protect consumer data, inflating its financial statements and share price before the truth became known.

The case was filed on Friday in federal court in Atlanta by the law firm Levi & Korsinsky, which provided a copy of the complaint. A copy could not immediately be located in court records.

Equifax’s share price closed down $10.11, or 8.2 per cent, at $113.12 on Monday. It has fallen 20.7 per cent in the two trading days since the 7 September disclosure of the breach, reducing Equifax’s market value by more than $3.5bn.