A man has been killed in a horse race at a birthday party that hit the headlines after an invitation went viral on Facebook.

About 1.3 million people vowed to attend a party for Rubi Ibarra Garcia in Mexico after her parents accidentally put out a public post on the social networking site.

The man died after stepping into the path of a galloping horse during a race in honour of the girl's 15th birthday in central Mexico's San Luis Potosi state.

Thousands of people are said to have attended after the Facebook post went viral. The party featured local bands and food.

The celebrations turned to tragedy though when Felix Pena, a 66-year-old man who regularly raced horses and worked at a local stable, was trampled during the race, according to the local attorney-general.

Local media said he owned one of the horses taking part in the amateur race.

In a video posted on local broadcaster Hora Ceros' website Pena had said he would enter his horse, Oso Dormido (Sleeping Bear), and that he was expecting to win, reported CNN.

"I'm not doing it for the money, I'm doing it for the fame and glory," he had said.

Another man was taken to hospital with a broken leg after falling into the horses' path, CNN said.

Medics at the scene say that Pena died en route to the hospital.

It is unclear why he stepped into the line of galloping horses. According to the local authority, an investigation has been opened.

Some bystanders said he may have misjudged the distance as he tried to cheer on Oso Dormido.

Mr Pena had been attracted to enter his horse for the race after seeing the Facebook invitation posted by Rubi's parents.

In a video message recorded earlier this month (Dec), Crescencio Ibarra announced that he would be organising a 15th birthday party for his daughter Rubi on Dec 26 in La Joya community, central Mexico.

"Hello, how are you? We invite you this 26 December to the 15th birthday party of our daughter, Rubi Ibarra Garcia," Mr Ibarra said while standing next to Rubi and his wife Anaelda Garcia.

He then went on to announce that the winner of the horse race to be held as part of the event would win 10,000 pesos (S$700).

Police said that apart from the accident during the horse race, the party went off peacefully.

Throwing a large party for a girl's coming-of-age is not unusual in Mexico, but Rubi Ibarra Garcia's party was unique in the attention it drew, said the BBC.

Mr Ibarra's message was posted publicly on Facebook and was widely shared on social media, prompting countless memes and imitations.

Mr Ibarra later said that the idea had been to invite neighbours and friends only, but promised that he would not turn anyone away.

On the day, thousands of people showed up and relatives had to move away an army of waiting reporters so Rubi could attend an open-air religious mass.

Dozens of tents and tables filled with food were prepared for those who made the journey.

They were met by Rubi, who looked serene despite the crowds, wearing a fuchsia dress and a tiara, said the BBC.

Her father was at one point exasperated by the media and threatened to end the party if reporters did not give his daughter some space.