First Bieber, then Dylan at Grammy rehearsals

Newcomers and seasoned musicians are taking turns on the Staples Center stage during Grammy rehearsals in Los Angeles.
First up Saturday was 16-year-old pop star Justin Bieber and his mentor, Usher. Bieber opened the performance with his acoustic guitar, then ditched it as he launched into an amped-up version of his new song, "Never Say Never."
Usher followed with his dance track "OMG."
Both singers were backed by pyrotechnic effects and a stage full of dancers.
There were no special effects during Mumford & Sons' performance of their hit, "The Cave." The seven-piece folk band played acoustically on a bare stage, followed by fellow folksters the Avett Brothers.

Both bands came together to back folk icon Bob Dylan, who sang his 1965 blues track, "Maggie's Farm."