The US Justice Department (DOJ) told a federal judge that Google illegally dominated online advertising technology, in seeking a second antitrust win against the company.

"Google rigged the rules of the road," said DOJ lawyer Aaron Teitelbaum, who asked the judge to hold Google accountable for anti-competitive conduct and added Google is "once, twice, three times a monopolist."

Google lawyer Karen Dunn said DOJ had not met its legal burden and was asking the judge to disregard antitrust and overrule key precedents.

"The law simply does not support what the plaintiffs are arguing in this case," Ms Dunn said.

The closing arguments in Alexandria, Virginia, cap a 15-day trial held in September where prosecutors sought to show Google monopolised markets of online advertising.

This trial in a Virginia federal court is Google's second ongoing US antitrust case as the US government tries to rein in the power of big tech.

In a separate trial, a Washington judge ruled that Google's search business is an illegal monopoly, and the US Justice Department is asking that Google sell its Chrome browser business to resolve the case.

This latest skirmish, also brought by the Justice Department, focuses on ad technology — the complex system determining which online ads people see when they surf the web.

Presiding Judge Leonie Brinkema has promised to deliver her opinion swiftly, as early as next month.

'Old' practices

The government alleged that Google controls the auction-style system that advertisers use to purchase advertising space online.

It was argued that this approach allows Google to charge higher prices to advertisers while sending less revenue to publishers such as news websites.

"This technology may be modern, but the practices (shown by Google) are as old as monopolies themselves," Julia Tarver Wood, a Justice Department lawyer, told the courtroom during the trial.

The US argues that Google used its financial power to acquire potential rivals and corner the ad tech market, leaving advertisers and publishers with no choice but to use its technology.

Another DOJ lawyer, Julia Tarver Wood, compared the case to the Charles Dickens novel A Tale of Two Cities, and said US Judge Leonie Brinkema had to decide whether to adopt the DOJ or Google version of the state of the ad market.

Google's parent company Alphabet is worth $US2 trillion ($3 trillion).

Google says it does not dominate

Google has dismissed the allegations as an attempt by the government to pick "winners and losers" in a diverse market.

The company argues that the display ads at issue are just a small share of today's ad tech business.

Google says plaintiffs ignore ads that are also placed in search results, apps and social media platforms and where, taken as a whole, Google does not dominate.

"The plaintiff's case is a little like a time capsule," Google's lawyer Karen Dunn said during the trial.

She warned that if Google were to lose the case, the winners would be rival tech giants such as Microsoft, Meta or Amazon, whose market share in online advertising "is ascendant as Google's share is falling."

Google also points to US legal precedent, saying arguments similar to the government's have been refuted in previous antitrust cases.

If Judge Brinkema finds that Google broke the law, she would consider prosecutors' request to make Google at least sell off Google Ad Manager, a platform that includes the company's publisher ad server and its ad exchange.

Whatever the judgement, the outcome will almost certainly be appealed, prolonging a process that could go all the way to the US Supreme Court.

Wires/ABC