Zimbabwe's opposition party says a top aide to its presidential candidate has been arrested upon returning to the country.Tendai Biti, secretary-general of the Movement for Democratic Change, had predicted his arrest as he prepared to return to Zimbabwe from South Africa earlier Thursday. Nqobizitha Mlilo says confirmation of the arrest came soon after Biti landed in Harare. Reporters awaiting Biti had seen security officers waiting at the Harare airport, but did not see Biti's arrest or see him enter the arrivals lounge.