Although the companies have yet to file any formal objections with regulators in the U.S. or Europe, they are beginning to publicly voice their concerns, according to a source close to one of the companies.
If the deal goes through, Google would account for 80 percent of the ads served on the Internet, the source said.
Google announced its plans toon Friday. Microsoft had also been in the running to acquire DoubleClick.
In a statement on Sunday, Microsoft General Counselcalled for regulators to give the deal a hard look.
"This proposed acquisition raises serious competition and privacy concerns in that it gives the Google-DoubleClick combination unprecedented control in the delivery of online advertising and access to a huge amount of consumer information by tracking what customers do online," Smith said. "We think this merger deserves close scrutiny from regulatory authorities to ensure a competitive online-advertising market."
Google could not immediately be reached for comment Sunday. A Yahoo representative declined to comment.
Google plans to buy DoubleClick from San Francisco-based private-equity firm Hellman & Friedman, which acquired DoubleClick two years ago for $1.1 billion, and JMI Equity. The deal is subject to regulatory approval.
David Drummond, senior vice president of corporate development at Google, said he is confident that antitrust regulators will approve the agreement.