Afghanistan's presidential election has been beset by fraud and corruption, a BBC investigation suggests

Voting cards are being sold openly and candidates have been offering thousands of dollars in bribes for votes.

The findings came as campaigning closed before Thursday's election, in which incumbent President Hamid Karzai faces 41 challengers.

A senior Afghan Independent Election Commission official denied to the BBC that voting cards were being sold.

The allegations came as militants launched a flurry of deadly attacks across Afghanistan on Tuesday, in the wake of their threats to disrupt the election.

Multiple voting cards

An Afghan working for the BBC went undercover in …