The US government has accused Google of manipulating the ad tech market, stifling competition and innovation; the company recently lost an antitrust case targeting its search business.

Google has monopolized the technology used to buy and sell online display ads by restricting or eliminating the choices of its customers — both website publishers and advertisers — the US Justice Department alleged at a federal antitrust trial.

Google opened its defense against allegations that it holds an illegal monopoly on online advertising technology Friday.

The government on Friday concluded its main arguments in the case -- US et al. v. Google -- and the internet giant started mounting its defense. The case, filed last year, accuses Google of building a monopoly over the technology that places ads on websites around the internet.