South Korean police have raided the national headquarters of Starbucks over defamation claims surrounding a controversial marketing campaign.

Starbucks Korea had in May launched a coffee tumbler promotion called “Tank Day” on the anniversary of the 1980 Gwangju Uprising, where hundreds were believed to have been killed by military forces.

Many felt the “tank” was a reference to vehicles deployed to crush pro-democracy protesters and the promotion sparked a mass boycott movement.

Starbucks quickly rolled it back, saying it was unintentional but civic groups filed a criminal complaint against the company, demanding the police investigate it for defaming victims of the military dictatorship.

Shinsegae Group, which operates Starbucks in Korea, said it had launched an internal investigation into the campaign and concluded that the mistake was unintentional.

It added the team that had planned the promotion did not draw the connection to the historical event, and the upper management had failed to spot it as well.

In June, Starbucks stores across the country closed for half a day to allow staff to attend history lessons.

Shinsegae apologised for the “inappropriate marketing” and said that the Tank Day controversy had led to serious decline in Starbucks sales in South Korea – one of the coffee chain’s largest markets.

The company also sacked its chief executive.