The Uzbek general, Abdul Rashid Dostum, has become the most high-profile challenger to Afghan President Hamid Karzai in the country's presidential election.
Dostum's decision will raise issues of ethnic divisions
General Dostum on Thursday resigned as President Karzai's military adviser in order to stand against him in the poll on 9 October.
"I will be a candidate in the presidential election," General Dostum told a rally of thousands of supporters in the northern city of Mazar-e-Sharif.
General Dostum is the main leader of Afghanistan's Uzbek community and his decision threatens to bring the divisive issue of ethnicity to the forefront of the election campaign.
President Karzai is a member of the country's largest Pashtun tribe.
An aide of General Dostum said the Uzbek leader was displeased with President Karzai's policies.
Akbar Bai told the Reuters news agency the general was unhappy at the "warlord" label applied to non-Pashtun commanders and the "unbalanced" campaign to disarm factional militias.
President Karzai has yet to respond to the general's decision to stand.
He is widely expected to win, but Mr Bai said Taleban violence in the south and east, the Pashtun heartlands, had harmed the registration process there
"Karzai will not be able to win, because the registration process is very slow in the south and east where he comes from, and we have lots of registrations in the north," Mr Bai said.
He said some of the 20-odd challengers to President Karzai had thrown their support behind General Dostum.
The general's controversial career has been marked by an ability to survive by switching sides in Afghanistan's complex web of shifting alliances.
He fought on the side of the ruling Soviets, then with ex-President Najibullah against the mujahideen who were battling the Soviet administration.
By 1997, Dostum controlled a kind of mini-state in the north
In 1992, the general then switched to the mujahideen side and by 1997 controlled a kind of mini-state in northern Afghanistan.
He fled the Taleban but returned to fight it as part of the Northern Alliance.
Since the fall of the Taleban, his militia has been involved in a number of bloody clashes for territory in the north, mainly against his arch-rival, the Tajik general, Atta Mohammad.
Although nominally a supporter of President Karzai, and a military adviser, General Dostum's alliance with the leader has been frail and he failed in a bid to become defence minister.
The general has refused to hand over heavy weapons under the national disarmament plan for factions.
At the talks for the country's new constitution in January he opposed the centralised state it enshrined.
The general was also accused of driving a Karzai-appointed governor from a north-eastern province in April.
President Karzai recently said warlords and their militias were a bigger threat than the Taleban.
The appointment of Atta Mohammad as governor of Balkh province on Tuesday will have alienated General Dostum further, although aides say this did not determine his election decision.
General Dostum called Atta Mohammad a "brother" at his Mazar rally and praised the defence minister, Mohammed Fahim, also a Tajik.
Some analysts believe General Dostum's best hope will be to deprive President Karzai of the 50% needed to avoid a run-off vote.
For now, General Dostum is keeping his message simple.
"Everyone has their dignity. I will defend your rights," he told his supporters.