* Summons comes after violent scuffle over tie-up

By Cyntia Barrera

MEXICO CITY, Feb 24 (Reuters) - Mexico's chief competition regulator gave testimony on an undisclosed issue after being summoned by prosecutors on Friday, weeks after a scuffle broke out when his agency tried to notify two companies about a blocked acquisition.

Eduardo Perez Motta, head of Mexico's anti-trust agency Cofeco, testified for at least eight hours to federal prosecutors, a Cofeco source said.

The source was not authorized to say what the citation was about and a spokesman for the attorney general's office could not immediately comment.

At the beginning of this year, a dispute exploded when Cofeco blocked Mexico's largest broadcaster, Televisa , from trying to buy half of cell phone company Iusacell for $1.6 billion.

Televisa saw the bid as a way to expand into Mexico's phone market, which is dominated by the world's richest man, Carlos Slim, and his company America Movil.

But Iusacell is a sister company of the country's No. 2 television network TV Azteca and Cofeco had concerns that ties between the two broadcast giants could lead to collusion in the advertising market.

When Cofeco representatives went to the Iusacell offices in Mexico City on Feb. 1 to notify the phone company about its decision, a violent scuffle broke out between security guards from both sides.

After the fight, the chief legal counsel for Iusacell's owner Ricardo Salinas said the company was considering filing a criminal complaint with the attorney general's office.

The Cofeco source could not confirm if Perez Motta was discussing the incident on Friday.

"Authorities have requested to keep a low profile because it is not over yet," the source said.

Cofeco's decision to block the Televisa-Iusacell tie-up could still be subject to revision, since both companies have the right to appeal.

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