The clash is broadly seen as a test of whether recent efforts to more aggressively curb the power of tech giants around the world will succeed. Lina Khan, the chair of the F.T.C., has argued that big tech companies have vast influence over online commerce and communication, allowing them to engage in anticompetitive practices that harm consumers.

“If this deal is completed, the combined company will have and is likely to have the ability and incentive to harm competition in various markets related to consoles, subscription services and cloud,” James Weingarten, the F.T.C.’s lead lawyer, said in court on Thursday.

Mr. Weingarten said Microsoft could make Activision’s games exclusive to its Xbox console, or degrade their quality on other platforms to make the Xbox more appealing to gamers. He pointed to Microsoft’s $7.5 billion purchase of ZeniMax Media and its slate of game studios in 2020, after which Microsoft made some of those games exclusive to Xbox. The F.T.C. pointed to an agreement made between Disney and ZeniMax to produce a game about Indiana Jones for multiple consoles. After Microsoft bought ZeniMax, the agreement was amended and the game became exclusive to Xbox. It was an indication, the F.T.C. appeared to be arguing, that Microsoft could make future Activision games exclusive to Xbox, even amending existing deals to do so.

The F.T.C. has argued that absorbing Activision’s games into Microsoft’s portfolio would also give it an unfair edge in the nascent market for cloud gaming.

Sony, which makes the rival PlayStation console, has been a vocal critic of the deal, and has said that PlayStation gamers could lose access to Call of Duty — an enormous franchise that has earned more than $30 billion in lifetime revenue — if Microsoft decided to make the game exclusive to Xbox. Microsoft has denied that it would do so.