Microsoft is keen to better compete with Google
Google has said it finds Microsoft's $44.6bn (£22.65bn) bid to buy rival Yahoo "troubling" and wants regulators to scrutinise the proposed deal.
In a blog, Google said the tie-up could unfairly limit the ability of consumers to freely access competitors' email and instant messaging services.
It said Microsoft had previously sought "to establish proprietary monopolies".
Microsoft made an unsolicited offer for Yahoo on Friday, and Yahoo has said it is considering the proposal.
Steve Ballmer, Microsoft's chief executive, told analysts at a briefing on Monday the proposed takeover would create a "strong number two competitor".
He argued that competition would get fiercer, particularly in the market for online advertising.
"Google's clearly got a dominant position. They've got about 75% of paid search worldwide," Mr Ballmer said.
But that view is not held by the top executives at Google.
"Microsoft's hostile bid for Yahoo raises troubling questions," said David Drummond, Google's senior vice president for corporate development and chief legal officer.
"This is about more than simply a financial transaction, one company taking over another. It's about preserving the underlying principles of the internet: openness and innovation," he said in a company blog.
Mr Drummond suggested Microsoft may attempt to exert an "inappropriate... influence" over the internet.
"While the internet rewards competitive innovation, Microsoft has frequently sought to establish proprietary monopolies - and then leverage its dominance into new, adjacent markets," he said.
In 2004 the European Commission fined Microsoft 497m euros for abusing its market dominance, a ruling the US company finally lost on appeal in September last year.
The Commission has since launched two new competition inquiries against Microsoft.
Members of the US Congress Judiciary Committee will scrutinise the bid on 8 February.
Borrowing to buy
Despite Microsoft's deep pockets, the software giant has revealed it may have to go into debt for the first time to finance its $44.6bn combined cash and share offer for Yahoo.
"If you look at the cash component, that's going to be over $20bn in cash," said Microsoft's chief finance officer, Chris Liddell.
"We could fund most of that through our cash holdings, but it's likely we're actually going to borrow for the first time."
Microsoft's proposed bid, unveiled in a letter to Yahoo's board on Friday, is 62% above Yahoo's closing share price on Thursday.
Reports said Yahoo would consider an alliance with Google as one way to fend off Microsoft's bid.
The Wall Street Journal reported on Sunday that Google's chief executive Eric Schmidt called his counterpart at Yahoo, Jerry Yang, to offer his company's help in any efforts to rebuff Microsoft.
Time Warner, News Corporation, AT&T and Comcast are some of the firms that are often named prospective suitors for Yahoo.
But, according to the New York Times, none of these companies have begun work on any rival bids.
Microsoft shares began Monday's trading session up slightly. At 1510 GMT it was up 8 cents at $30.53.