By Paul Carsten

ABUJA (Reuters) - Nigerian protesters vandalised the head office of South African mobile phone giant MTN in Abuja on Thursday in apparent retaliation for anti-Nigerian violence in South Africa, a witness and an MTN spokesman said.

The attack came after a Nigerian youth body threatened to shut down South African businesses after shops believed to be owned by immigrants in the capital Pretoria were looted.

Nigeria and South Africa, the continent's two largesteconomies and pre-eminent diplomatic and military powers, havea volatile relationship.

Protesters stormed the regional head office of MTN - the biggest South African company active in Nigeria - and stole customers' phones, vandalised equipment and attacked customers, a spokesman for MTN said.

A Reuters reporter arriving at the scene after the storming could see no obvious damage at the building in the centre of Nigeria's capital Abuja. Security officers had cordoned off the entrance.

"They forcefully entered the MTN office. Security men were around but unable to curtail the protesters, who forced the gates open and entered the office," a witness said.

"Some protesters were touts. Some were students. In the midst were some criminals that took away some customers' phones and other things," he added, asking not to be named.

A South African government source described the attack asserious. "This wasn't just some people on the street throwingstones. They broke into the building and stole things and brokethings," the source said. The incident coincided with a visit by MTN chairman PhuthumaNhleko to the Nigerian capital to see Vice President YemiOsinbajo, who has been in charge for the last month for President Muhammadu Buhari, who is absent on sick leave.

This week, at least 20 shops believed to belong toimmigrants were looted in South Africa's capital. Policerefused to say if the attackers were specifically targetingforeigners.

Responding to those attacks, Nigeria's foreignministry said it would summon South Africa's envoy to raise itsconcerns over "xenophobic attacks" on Nigerians, other Africansand Pakistanis.

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South Africa's home affairs minister on Thursday spoke out against renewed anti-foreigner violence.

Anti-immigrant violence has flared sporadically in South Africa against a background of near-record unemployment, with foreigners being accused of taking jobs from locals and getting involved in crime.

"Currently, in 2017, there are renewed incidents of violence against foreign national in Rosettenville and Pretoria West," Gigaba told a media briefing.

"Unfortunately, xenophobic violence is not new in South Africa," he said, recounting similar incidents in the past.

The minister said some residents in Pretoria have planned a march on Friday against immigrants, citing competition for jobs and allegations of criminal activity, such as prostitution and drug dealing in the poor township west of Pretoria.

(Reporting by Ed Cropley in Johannesburg, Chijioke Ohuocha in Lagos, Paul Carsten in Abuja and Wendell Roelf in Cape Town; Writing by Ed Cropley and Ulf Laessing; Editing by James Macharia)