* Empty streets, closed shops in flashpoint city Kaduna
* Protests in Rivers, Bauchi states
* Concerns over collation process (Adds results from key states Kaduna, Kano)
By Tim Cocks and Alexis Akwagyiram
ABUJA, March 30 (Reuters) – Nigerian presidential challenger Muhammadu Buhari recorded thumping majorities in key northern states on Monday, as the United States and Britain expressed concerns about meddling with the vote count.
Buhari, a 72-year-old former military ruler who has campaigned as a born-again democrat intent on cleaning up the corrupt politics of Africa's most populous nation, won 1.1 million votes in the flashpoint city of Kaduna.
President Goodluck Jonathan, a 57-year-old southern Christian, won 484,000 votes there.
The city, scene of three days of bloodletting after Buhari lost to Jonathan in the last election in 2011, was tense but quiet as the results trickled in, with the roads empty of traffic and many shops and homes shuttered.
Buhari, a northern Muslim, also won 1.9 million votes in Kano against 216,000 for Jonathan, an indication of the political polarisation that has deepened over the last five years under Jonathan's People's Democratic Party (PDP).
Although the economy has been growing at 7 percent or more, scandals over billions of dollars in missing oil receipts and the eruption of an Islamist insurgency in which thousands have died have undermined Jonathan's popularity.
Although the early results will hearten the Buhari camp, they are far from conclusive in an election forecast to be the closest since the end of military rule in 1999.
In his native Rivers state, the volatile and hotly contested home of Africa's biggest oil and gas industry, Jonathan won a massive 95 percent of the vote.
The results coming from states such as Rivers have prompted suspicion among diplomats, observers and the opposition, whose sympathisers took to the streets in protest.
Police fired tear gas at a crowd of 100 female supporters of Buhari's All Progressives Congress (APC) demonstrating outside the regional offices of the INEC election commission.
"Their intention was to destroy INEC materials," a policeman at the scene told Reuters.
The weekend vote was marred by confusion, technical glitches, arguments and occasional violence but in many places proved to be less chaotic than previous elections in Africa's biggest economy and top oil producer.
At least 15 people were shot dead on polling day, most of them in the northeast where the islamist insurgent group Boko Haram has declared war on democracy in its fight to revive a mediaeval caliphate in the sands of the southern Sahara.
However, the United States and Britain said that after the vote there were worrying signs of political interference in the centralised tallying of the results.
"So far, we have seen no evidence of systemic manipulation of the process," U.S. Secretary of State John Kerry and British Foreign Secretary Philip Hammond said in a joint statement.
"But there are disturbing indications that the collation process – where the votes are finally counted – may be subject to deliberate political interference," they added.
Such views are likely to fuel an APC belief of political skulduggery, and increase the chances of a repeat of the 2011 post-election violence in which 800 people were killed, most of them in the predominantly Muslim north.
Even before preliminary tallies were recorded on Sunday, the party rejected the outcome in Rivers state and denounced the vote there as "a sham and a charade".
PDP also painted itself as a victim of rigging, but said it would make no difference.
"We are confident of victory," party spokesman Femi Fani-Kayode told journalists. "Any attempt to manipulate figures or to rig us out from any quarter will be firmly resisted."
World powers and international investors are watching the conduct of the poll closely to see whether one of Africa's most important states can improve its patchy record.
Fitch cut Nigeria's credit outlook to negative on early on Monday, but kept its BB- rating, citing the political uncertainty.
However, the stock market climbed 1.7 percent, narrowly missing a three-week high in its seventh consecutive day in the black as domestic investors bet on a relatively smooth outcome.
Bonds also gained, dealers said. The naira traded at 218 against the dollar on the black market, roughly in line with its levels before the election.
"If there's no post-election violence, the market will rise," said Ayodeji Ebo, head of research at Afrinvest in Lagos.