"If you are trying to register as a supporter and are experiencing difficulties, the deadline has been extended to 3pm if you want to vote," the party said on its Twitter page as thousands of people tried to sign up.
Frontrunner Jeremy Corbyn earlier made a last-minute call on his supporters to vote for him.
Corbyn, 66, an anti-austerity candidate, earlier suggested he might pursue a closer relationship with Russia if elected leader.
Corbyn’s rivals include Andy Burnham, Yvette Cooper and Liz Kendall, Shadow Secretary of State for Health, Shadow Home Secretary and Shadow Minister for Care and Older People respectively.
The leadership race was triggered by Ed Miliband stepping down as leader after his party lost in the May general elections.
The Labour leadership elections will conclude on September 10. The results are expected to be announced on September 12.
