Pharrell performs at Apple campus in celebration of Earth Day, dons Apple Watch onstage

Source: Tim Cook via Twitter

Apple CEO Tim Cook tweeted out a picture of the performance, captioned, "Thanks @Pharrell for an unforgettable Earth Day celebration," using the music icon's Twitter handle. Others in attendance also posted photos of the event to social media sites, including one showing Pharrell holding up his Apple Watch Edition to the crowd.

Pharrell was among the first high-profile celebrities to be seen wearing Watch in public well before public availability, in his case at a taping of NBC's talent show "The Voice" in early April. He later uploaded a short Instagram video showing off the device configured with a Mickey Mouse watch face.

Since then, the marketing campaign has extended to Katy Perry, Drake and British rugby star Will Carling. Most recently, Beyonce was spotted wearing a yellow gold Apple Watch Edition with custom gold link bracelet, a combination first seen on the wrist of fashion designer Karl Lagerfeld.

Today's concert is part of a wider Apple campaign touting the company's environmental responsibility initiatives. Earlier today, for example, employees at Apple Stores around the world wore green shirts and put up special signage, changing the leaf in Apple's iconic logo green.