There has been a big blast in the west of the Afghan capital, Kabul, near the compound where a security pact with the US is to be debated, police say.
Reports suggest it may have been a suicide bomber in a car. Police say there were casualties.
The security pact governs the status of the 10,000 US military personnel who will stay in Afghanistan beyond 2014.
It is being discussed next week by Afghanistan’s Loya Jirga, the traditional council of elders.
A police chief, quoted by the AFP news agency, spoke of an explosives-laden car and one witness described seeing a white vehicle in the area before the blast.
The blast reportedly occurred less than 100 metres from a large tent where more than 2,000 prominent Afghans gather on Thursday to debate the pact.
The security agreement must be approved by the Loya Jirga and parliament before it can come into force.
Among other issues, the pact covers the key issue of whether US troops can be prosecuted in Afghanistan.
Hours before the blast, Mr Karzai said negotiators had completed a draft of the accord, AP reports.
But he added that there were “still some differences”.