Symantec, the computer security firm, said it intercepted around 85 targeted attacks a day in March, including efforts to steal personal information through phishing or with links to nefarious software that could ultimately expose corporate files. The only month with more attacks was March 2009, when there was a surge that coincided with a G20 summit meeting.

Symantec said the most common targets were government agencies and senior managers and executives; the phishing of such big game is commonly referred to as “whaling.” Manufacturing firms were the targets of 15.9 percent of the attacks, compared with 8 percent for the financial sector and 6.1 percent for technology companies, Symantec said. Hackers taking aim at corporations are often seeking new product designs and may focus on engineers at a defense contractor, for example, to get data they can sell on the black market.

Enrique Salem, Symantec’s chief executive, gave the example of an e-mail sent to the head of a company that appears to be from the Internal Revenue Service. The message raises questions about the tax implications of an acquisition, and the chief executive passes the message to others inside the company. Someone opens the attachment, giving the attacker access to the company’s internal network.

“It’s about getting you to do something to compromise the system,” Mr. Salem said.

In the case of the Gmail attacks, Google said they appeared to originate from Jinan, China, and were aimed at users like Chinese political activists, military personnel, journalists and South Korean officials.

The Chinese Foreign Ministry said Thursday that the government had no involvement in any such attacks, and that it “consistently opposes any criminal activities that damage the Internet and computer networks including hacking, and cracks down on these activities according to law.”

It is not clear how the attackers obtained the Gmail addresses they used, although they could have been found inside other compromised accounts, including corporate or government accounts whose addresses are often easier to guess.