Berlin - Voting booths opened for Germany's general election at 8am (0600 GMT) on Sunday, amid expectations that Chancellor Angela Merkel of the Christian Democrats (CDU) would win a renewed mandate, ahead of Social Democrat (SPD) challenger Frank-Walter Steinmeier.

Merkel, whose CDU is expected to pick up the largest share of the vote, is hoping for enough support to enter a new centre-right coalition with the Free Democrats (FDP).

Failing this, the most likely outcome would be a continuation of the "grand" centre-left coalition with the SPD.

In the final hours of the election campaign the major parties were trying to win over the undecided, as up to a third of those surveyed said they had not made up their mind.

"It is worth having a word with every neighbour and friend today, right into the late hours of the night," Merkel told 2,500 supporters on Saturday at a CDU rally in Berlin.

In Dresden, Steinmeier called for a vigorous campaign until the moment the voting booths closed. "The election result will be very different from what was predicted weeks ago," the SPD candidate said.

After 11 years in government, polls predicted that the SPD would face its worst result since 1945.

Meanwhile FDP leader Guido Westerwelle relished what he called his "last hours of opposition," in the hope that 11 years outside government were drawing to an end.

In total, 62.2 million Germans are entitled to vote for 28 parties seeking to enter the German parliament, or Bundestag.

Only five parties - CDU, SPD, FDP, Greens and the Left Party - are expected to clear the 5-per-cent hurdle to fill the parliament's more than 600 seats.

Poll results could be skewed by an electoral quirk which means parties can win additional, or "overhang" seats in parliament, over and above their proportional allocation of votes.

This system, which tends to benefit the largest parties, could see the CDU gain up to 20 additional seats - enough to feasibly create a majority for a CDU-FDP coalition.

In two federal states, voters are also electing their regional governments. This also affects the make-up of the upper house of parliament, or Bundesrat.

A close race between the CDU and SPD is expected in both the northernmost state of Schleswig-Holstein and the eastern state of Brandenburg, which surrounds Berlin.

Security services were on high alert after a series of militant- Islamist videos threatened terror attacks if Germany did not withdraw its troops from the NATO-led mission in Afghanistan.

An interior ministry spokesman said all seven recent videos were being treated very seriously, but also in a "calm and diligent" manner.

Merkel has centered her election pledges on the economic crisis, arguing that the need to stimulate economic growth would be best met by a coalition with the free-market FDP.

The SPD has drawn up an ambitious plan to create 4 million new jobs, half of which are to come through so-called green technology.

The CDU and FDP also plan to extend the life spans of existing nuclear reactors, which the SPD and Greens want to see phased out by 2020.

The Left Party - grown out of former East Germany's communist party - still has a strong base in the eastern states and has drawn support from ex-SPD voters disgruntled at the SPD's centrist policies.

Parties can no longer afford to ignore Germany's ageing population, both for its impact on the country's welfare state, as well as the fact that voters over the age of 60 make up a third of the electorate.

In the last election, in 2005, the CDU won 35 per cent of the vote while the FDP scraped 10 per cent, falling short of opinion poll predictions of a CDU-FDP majority. At the time, the SPD won 34 per cent, the Left achieved almost 9 per cent, and the Greens trailed at 8 per cent of the vote.

Polls close at 6pm (1600 GMT), at which time preliminary exit poll results are due. (dpa)