Voters have been turning out in numbers in Venezuela's referendum on far-reaching constitutional changes sought by President Hugo Chavez.
The raft of proposed reforms would see the end of presidential term limits and the Central Bank's autonomy removed.
Mr Chavez says the proposed changes would return power to the people, but opponents accuse him of a power grab.
Residents of the capital, Caracas, were woken before dawn by fireworks and loud music, says a BBC correspondent.
It was a rallying call to vote, correspondent James Ingham says, and appeared to have the desired effect, with long queues, several hours long, forming outside polling stations.
The past weeks have seen large anti-reform protests and the defection of several Chavez allies, complaining that his reforms go too far.
Mr Chavez has claimed that the opposition, with the support of the Bush administration in Washington, may try to sabotage the vote through violent protests.
US officials have called Mr Chavez' claims that Washington is conspiring to topple his government, and possibly backing plots to assassinate him, ridiculous.
Mr Chavez says the package of reforms is necessary to "construct a new socialist economy".
He has proposed 33 changes, and the National Assembly, which is composed of his supporters, put forward a further 36 amendments.
Mr Chavez' opponents have called for close monitoring of the ballot. Opinion polls have suggested that the result could be close, although surveys in the past have tended to underestimate the level of support Mr Chavez enjoys.
The BBC 's Americas editor, Emilio San Pedro, says the elections are expected to be as free and fair as all previous ones since Mr Chavez came to power in 1998.
One proposal is to allow the president to stand for re-election an indefinite number of times.
Speaking on Friday, Mr Chavez said: "If God gives me life and help, I will be at the head of the government until 2050." He would be 95 years old.
Under the current constitution, Mr Chavez would have to stand down when his term expires at the end of 2012.
Other changes up for approval include giving the president control over the central bank, the creation of new provinces governed by centrally-appointed officials, and a reduction in the voting age from 18 to 16.
There are also proposals to expand presidential powers during natural disasters or political "emergencies".
On the social front, changes include establishing a maximum six-hour working day and 36-hour working week, and widening social security benefit to workers in the informal economy.
A number of defections from the president's camp have encouraged opponents, but Mr Chavez has dismissed these one-time allies as traitors.