Google announced the deal on Friday, after beating bids from Microsoft and Yahoo! with an all-cash offer, allowing DoubleClick's main shareholder, private equity house Hellman & Friedman, to exit the company it bought in July 2005.
The winning bid exceeded ballpark valuations of DoubleClick by $1bn.
Eric Schmidt, chief executive of Google, said the combination with DoubleClick would "accelerate the adoption of Google's innovative advances in display advertising".
However, Microsoft criticised the tie-up, claiming that Google and DoubleClick account for more than 80% of the ads delivered to website publishers.
Brad Smith, Microsoft's general counsel, said: "We think this merger deserves close scrutiny from regulatory authorities to ensure a competitive online advertising market."
Fears about the power Google will now wield were also raised by Jim Cicconi, AT&T's head of external and legislative affairs, who said "we would be forced to deal with Google on Google's terms".
Yahoo! and AOL have also privately expressed concern about the deal, according to newspaper reports today.
The deal will need to be approved by US and European competition regulators.
Separately, Google made progress in its attempt to expand into selling ads in other media over the weekend. Yesterday, it secured a deal to sell a share of the ad inventory for US radio company Clear Channel.