It has been a high-pressure campaign and Indonesians are preparing to vote in their parliamentary elections.

There are more than 6,600 candidates vying for 560 seats in the House of Representatives and 132 in the Upper House.

Another 16,000 hopefuls are competing at the provincial and district levels, making it arguably the world's largest single-day election process.

Recent surveys put the PDI-P ahead with about 20 per cent of the vote, while the Democratic Party of President Susilo Bambang Yudhoyono is struggling and expected to fare badly.

The Democratic Party won the 2009 presidential and legislative election in landslides but its advantage has been lost through the corrosion of corruption scandals, policy paralysis and a major slide in the President's popularity.

Still the Democrats believe they can secure 15 per cent of the seats and are polling in third place behind PDI-P and Golkar, once the political machine of the late dictator Suharto.

The race between PDI-P and Golkar will be closely run.

Meanwhile popular Jakarta Governor Joko Widodo has been confirmed as the main opposition candidate for the presidential election due in July.

Megawati Sukarnoputri, chairwoman of the Democratic Party of Struggle (PDI-P) finally gave her blessing to Jokowi, as he is commonly known, on March 14.

He is considered almost certain to be Indonesia's next president.

But to stand in his own right his party must first win 25 per cent of the popular vote or 20 per cent of the seats in the parliamentary elections.

If not the PDI-P must team up with another party.

Speculation is rife that Jokowi could campaign later in the year with former vice president Jusuf Kalla from the Golkar Party.

Susilo Bambang Yudhoyono cannot stand again because he has served the maximum two consecutive terms.

The election has seen investors hold off making decisions and economic growth is expected to slow this year and next.

GDP growth has slowed steadily in South East Asia's largest economy since 2011.

On March 14, the central bank, the Bank of Indonesia, lowered its 2014 growth forecast from the range of 5.8 to 6.2 per cent to 5.5 - 5.9 per cent as domestic consumption and exports prove to be less robust than previously estimated.

The Bank also thinks the impact of the two rounds of elections won't be as strongly positive as in previous elections.

There was an immediate reaction late Friday after Widodo was officially nominated as the PDI-P presidential candidate.

Indonesian stock prices jumped 3.2 per cent and the Rupiah strengthened against the US dollar.

There are 12 parties contesting the parliamentary elections in three weeks time.

At least one opinion poll suggests Islamic-based parties, or a coalition of some of them, could gain a significant number of votes.

Five Muslim-based parties are running: the United Development Party (PPP), the National Awakening Party (PKB), the Prosperous Justice Party (PKS), the Crescent Star Party (PBB) and the National Mandate Party (PAN).

Previous general elections have been virtually violence free, but violence has marred early campaigning in the autonomous province of Aceh where three local parties are running.

In a worrying sign, two candidates of the Aceh National Party have been killed in the lead up to the official start of the campaign.