A pregnant 15-year-old girl has undergone an ultrasound on stage during an anti-abortion rights protest in Mexico City, where some 10,000 demonstrators prayed and shouted slogans amid the spectacle.

Sunday's March for Women and Life, which wound from the Paseo de la Reforma to a rally in front of the city's iconic Angel of Independence monument, was made up mainly of Catholic groups pushing back after last month's Supreme Court ruling that would decriminalise abortion in Mexico.

"The government ... is elevating the right to abortion as a right to kill," 56-year-old protester Alma Bello told the AFP news agency. 

"It worries us a lot because it is not the feeling of a majority of Mexicans."

A stretcher was rolled out onto the rally stage for gynecologist Fernando Urquiza to perform an ultrasound on 15-year-old Ana, who is 38 weeks pregnant.

'Ready to be born'

Images of the inside of the teenager's uterus were broadcast on huge screens on either side of the stage, accompanied by cheers and applause from the crowd.

"All good to go, ready to be born," pronounced the doctor, who said he was "very excited" to be part of the display.

When asked how she was feeling during the exam, Ana replied only that she was "fine."

Meanwhile, a rally announcer told Ana the event was "the biggest baby shower I have ever seen."

Alison Gonzalez, a Catholic activist and head of Steps for Life, the group that organised the march, said the gathering was not meant as a response to any particular event –– such as the Supreme Court ruling –– but rather as a show of "national support for women."

"We need policies that reconcile the professional with the maternal, that make sure we can return home safely, that help us move forward in the face of an unwanted pregnancy," 26-year-old Gonzalez told AFP.

"Legal or illegal, abortion must be out of the question, because women deserve so much more," she said.

'Legal or illegal, abortion still kills'

In a highly orchestrated move, groups arrived in buses from far-flung cities such as Morelos in central Mexico and Jalisco in the west –– attendees all carrying hundreds of banners, scarves and signs in the signature light blue color of the international "pro-life" movement.

Some factions prayed Hail Marys together, while others shouted chants such as "Legal or illegal, abortion still kills!"

READ MORE: Police in Mexico clash with pro-abortion rights protesters