Attackers blew up and then stormed the political office of Amrullah Saleh, a former Afghan spy chief turned politician who is now the running mate of Afghan President Ashraf Ghani.
The blast hit the office in central Kabul on the first day of campaigning for presidential elections in September.
It was not immediately clear if Mr Saleh, who has been a vociferous opponent of the Taliban and Pakistan, was among those injured.
Mr Saleh led Afghanistan’s NDS intelligence agency before joining politics with the Green Trend movement.
“At around 4:40 pm (1210 GMT), first a blast occurred near Green Trend office… then a number of attackers entered that office,” interior ministry spokesman Nasrat Rahimi told AFP.