* Kerry warns against "parallel government" formation
* Afghanistan relies on foreign aid, particularly from U.S.
* Abdullah tells supporters he is the winner (Edits, adds Abdullah quotes, details)
By Mohammad Aziz and Mirwais Harooni
KABUL, July 8 (Reuters) - The United States warned on Tuesday it would withdraw financial and security support from Afghanistan if anyone tried to take power illegally, as supporters of a presidential candidate rallied in Kabul for a parallel government.
Preliminary results announced on Monday showed that Ashraf Ghani, a former World Bank official, won the June 14 second round, but his rival Abdullah Abdullah immediately rejected the outcome, saying the vote was marred by widespread fraud.
Underscoring the magnitude of the crisis, Abdullah said U.S. Secretary of State John Kerry would visit Kabul on Friday. Kerry arrived in Beijing on Tuesday for the annual Strategic and Economic Dialogue. The U.S.-China talks finish on Thursday.
Thousands of Abdullah supporters gathered in the capital on Tuesday, demanding their leader form a parallel cabinet and unilaterally assert his own rule - a dangerous move that would further fracture the fragile country.
In a sharp warning, U.S. Secretary of State John Kerry said there was no justification for violence or "extra-constitutional measures".
"I have noted reports of protests in Afghanistan and of suggestions of a 'parallel government' with the gravest concern," he said in a statement issued by the U.S. embassy in Kabul.
"Any action to take power by extra-legal means will cost Afghanistan the financial and security support of the United States and the international community."
Afghanistan is heavily reliant on foreign donors to fund everything from building roads and paying school teachers to security. The United States pays the lion's share of all international aid.
Observers fear that a standoff between Abdullah and Ghani could plunge Afghanistan into disorder, with no clear leader in a country already beset by deep-rooted ethnic divisions.
Abdullah has accused President Hamid Karzai, who is stepping down after 12 years in power, of helping rig the vote in favour of Ghani, describing it as a "coup" against the people.
The standoff over the vote has quashed hopes for a smooth transition of power in Afghanistan, a concern for the West as most U.S.-led forces withdraw from the country this year.
There are concerns however about how much Abdullah, who is popular among the powerful Tajik community in the north, would be able to control his supporters if the crisis escalated.
Thousands of his supporters vented their anger in central Kabul on Tuesday, chanting "Death to Karzai", tearing down a large portrait of the outgoing leader and replacing it with an image of Abdullah.
At the rally, Abdullah, visibly flustered by the size of the gathering, faced a roar of slogans demanding he immediately announce his own cabinet, telling supporters to be patient.
"We are the winner of this round of elections without any doubt," he told the flag-waving and whistling crowd.
"The people of Afghanistan call on me to announce my government today. This was and is a demand from the people of Afghanistan... We cannot ignore this call. ... Once again I ask you to give me a few days days to consult and speak."
The apparent softening of his tone comes after speaking with by telephone with Kerry and U.S. President Barack Obama.
"The main point in both U.S. Secretary of State's and the President's discussions was that John Kerry will come to Afghanistan on Friday and their promise was that they will be next to the people of Afghanistan in defending justice, fighting against fraud and revealing fraud," Abdullah said.
U.S. officials were not immediately available for comment.
Away from the city's centre, dozens of Abdullah supporters tore down another Karzai portrait at Kabul's international airport.
The Independent Election Commission announced on Monday that Ghani won the second round with 56.44 percent of the vote, according to preliminary results. The tally might change when the final official numbers come out on July 22.
Ghani, who is due to address reporters later in the day, is backed mainly by Pashtun tribes in the south and east of the country. (Writing by Maria Golovnina; Editing by Jeremy Laurence)