Media playback is unsupported on your device Media caption Starbucks staff are getting "unconscious bias" training, but what is it?

Coffee chain Starbucks has shut all 8,000 company-owned branches in the US for an afternoon to carry out "racial bias" training.

About 175,000 employees are taking part. The aim is to prevent discrimination in Starbucks cafes.

The move comes after the firm had to apologise over last month's arrest of two black men who were waiting to meet someone in a Starbucks in Philadelphia.

Following the incident, protesters have been calling for a Starbucks boycott.

Starbucks's CEO Kevin Johnson apologised on behalf of the company, promising to take action.

What about Tuesday's training?

Starbucks cafes shut across the US at about 14:00 local time.

Employees are being taught how to handle unconscious bias, in an effort to ensure all customers are treated fairly.

The closure could cost an estimated $20m (£15m) in lost sales.

Image copyright EPA Image caption A sign alerts customers to the day's early store closing time at a Starbucks coffee shop

Employees will be watching videos about bias featuring company leaders and rapper Common, the company said in a statement to preview the "curriculum".

"We realise that four hours of training is not going to solve racial inequity in America," Starbucks Chairman Howard Schultz told CNN on Tuesday.

"We need to have the conversation. We need to start."

In an open letter to customers on Tuesday, Mr Schultz thanked them for their "patience and support as we renew our promise to make Starbucks... an inclusive gathering place for all".

Media playback is unsupported on your device Media caption Watch police arrest two black men at Starbucks - and the protests that followed

At the same time, about 6,000 licensed Starbucks cafes will remain open at airports and grocery stores. Those working there will be trained later.

The merits of Tuesday's training are hotly debated, the BBC's Rajini Vaidyanathan reports.

Some have praised Starbucks, but others see it as a publicity stunt which will do little to end the persistent problem of racial profiling in the US, our correspondent says.

What happened last month in Philadelphia?

Young black entrepreneurs Rashon Nelson and Donte Robinson were waiting in a local Starbucks to meet someone and had not bought anything when the store manager called police.

Image copyright CBS Image caption Rashon Nelson (L) and Donte Robinson (R) were held in police custody for eight hours

Amateur video showed police placing the pair, who were accused by shop staff of trespassing, in handcuffs.

"All the other white people are wondering why it's never happened to us when we do the same thing," tweeted customer Melissa DePino, who posted a video of the incident.

Mr Johnson said the video was "hard to watch" and that the actions taken were "wrong" and "reprehensible".

Casting a wider racial spotlight

Analysis by Rajini Vaidynathan, BBC Washington

Just after 2pm the last customers were ushered out of this Starbucks store in Washington DC, and the doors were locked.

That hasn't stopped a steady flow of people arriving for their afternoon caffeine fix - they were instead greeted by a sign on the door explaining that for the rest of the day staff are getting training to make Starbucks "a place where everyone feels that they belong".

Some have dismissed the racial awareness session as nothing more than a publicity stunt, but most customers at this store earlier in the day felt it was a step in the right direction.

The treatment of Donte Robinson and Rashon Nelson at the Starbucks in Philadelphia last month has undoubtedly cast a spotlight on the wider issue of racial profiling in this country.

But for many black Americans it was unsurprising - being made to feel unwelcome in public spaces has been an unpleasant part of daily life for decades - one which an afternoon's training won't be able to fix.

Mr Nelson and Mr Robinson later reached a financial settlement with Starbucks and the city of Philadelphia.

They agreed to receive a symbolic $1 in damages. City officials also promised to set up a scheme for young entrepreneurs.

The deal includes a vow from the city to contribute $200,000 (£147,000) to its funding.

The store manager who called police has since left the company.