President Hugo Chavez is set to urge voters to back constitutional reforms he says are needed to further Venezuela's socialist revolution.
His call for a "Yes" vote in Sunday's referendum is due to be made at a rally in central Caracas later in the day.
The reforms include allowing the indefinite re-election of the president and ending the Central Bank's autonomy.
Critics say the reforms amount to a power grab but supporters say the changes will deepen democracy.
On Thursday, a huge crowd gathered in the capital to voice their opposition to the planned changes.
Supporters of the "Yes" campaign, who have begun taking to the streets, are due to hold their final rally in the same spot in central Caracas.
On Sunday, voters will be asked whether they agree with a package of constitutional reforms.
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.
Mr Chavez proposed 33 changes, and the National Assembly, which is composed of his supporters, put forward a further 36 amendments.
One proposal is to allow the president to stand for re-election an indefinite number of times.
Mr Chavez has said he is prepared to serve for life as long as the people want it. Under the current constitution, he 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 reducing the voting age from 18 to 16.
There are also proposals to expanding 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. ( BBC )