KABUL, June 21 (Reuters) - Protesters gathered in the Afghan capital on Saturday in support of presidential candidate Abdullah Abdullah's accusation of vote fraud, and his call for the organisers to stop counting the ballots from the run-off election held a week ago.

Several hundred people gathered, disrupting traffic on the main road to the airport, as the police were deployed with batons and riot shields to counter any violence.

Tensions have been running high in Kabul since the run-off between Mr Abdullah and his rival Ashraf Ghani, a former World Bank economist.

Mr Abdullah says he no longer trusts the electoral bodies and blames them and interference by President Hamid Karzai for mass fraud. He wants the electoral bodies to halt counting votes and has invited the United Nations to intervene.

The move has imperilled Afghanistan's political transition at a critical time, with most foreign troops preparing to leave by the end of 2014, and a Taleban insurgency still raging after 13 years.

Mr Karzai late on Friday - to the dismay of diplomats who want the electoral bodies to be respected and the election process to completed - issued a statement supporting Mr Abdullah's call for the United Nations to intervene in the election.

The United Nations has said it needs to hear the details of any proposal, but is ready to support an "Afghan-led process".

There are fears that street protests could turn violent and take on an ethnic dimension, as most of Mr Abdullah's supporters are Tajiks, the second-largest ethnic group, while Mr Ghani's supporters are mainly Pashtun, the largest group in Afghanistan.

Mr Abdullah's supporters chanted "we will defend our vote to the last drop of blood", while blocking the road leading to the airport on the outskirts of the capital.

They also brandished banners with slogans, including "Death to Karzai", "Death to Ghani". The Independent Election Commission (IEC) was similarly targeted, with banners that read "Death to the IEC".

Elsewhere in Kabul, a suicide bomber killed himself and a bystander in an attempt to assassinate an adviser to the the High Peace Council, which is seeking reconciliation with Taleban fighters.

The adviser, Mr Mohammad Massom Stanikzai, escaped unharmed, but other several people were wounded, police officials said.