Around 13,000 candidates are fighting for more than 4,000 seats on 159 municipal councils in England and Wales as well as the 25-member London Assembly.
The successful London mayoral candidate will administer an annual budget of more than $22bn and authority to make policy decisions that affect 7.5 million Londoners and the millions more who visit the capital city.
A victory for Johnson over Livingstone would be a symbolic boost for the Conservative party at a time when they are riding high in opinion polls.
A third consecutive four-year term for Livingstone, however, could reassure Labour that their recent dip in form is only temporary and they can recover before the country goes to the polls sometime before May 2010.
Brown has himself recognised that the government has faced a hard time as the impact of the global credit crunch begins to hit the housing market and economic growth, alongside rising food and fuel prices.
Political analysts predict that Labour will do well to better their performance the last time the seats were contested in 2004, when they came third in the national vote equivalent with a 30 per cent share of the vote.
At the time, the fall-out from Britain's involvement in the widely unpopular Iraq war hit them hard, but the party still recovered to win the 2005 general election, albeit with a reduced parliamentary majority.
This time, domestic issues, particularly the government's recent economic record, are likely to be the dominant theme.
There was widespread opposition to the use of billions of pounds of taxpayers' money to prop up the Northern Rock, which collapsed in the wake of the global credit crunch and was eventually nationalised in February.
The government's abolition of the 10 per cent tax threshold for lower paid workers has also caused outrage in Labour ranks, with claims the party had abandoned its core principles of helping the most needy in society.
Brown has admitted he made mistakes over the tax reform, and following threats of a party revolt, he made concessions on the policy last week.
Barring recounts, results for 100 of the 159 councils are expected late Thursday into early Friday.
The remainder are due from 1100 GMT on Friday, with London's next mayor one of the last to be declared.