Google opened its defense against allegations that it holds an illegal monopoly on online advertising technology Friday with witness testimony saying the industry is vastly more complex and competitive than portrayed by the federal government.

As the government wraps up its case, it has built a picture of how Google has become dominant among companies that buy and sell ads online.

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.

The judge who will decide whether Google holds a monopoly over technology that matches buyers and sellers of online advertising must choose whether to believe what Google executives wrote or