The SNP say they have mobilised an army of 30,000 activists to help get their vote out in the biggest polling day operation Scotland has ever seen.

Party sources said nearly one in three of the party's 105,000 members has been involved knocking on doors or phoning voters to encourage them to cast their ballot.

Labour said they had 2000 activists working across the country, as all the parties mounted their polling day "GTVO" or get-the-vote-out operations.

All use sophisticated electronic databases to record known supporters and contact them on election day.

According to a Labour source, 38 per cent of voters had cast their ballot by 5pm, a brisk rate but considerably lower than the 57 per cent who had voted by the same time in last year's independence referendum.

Organisers were expecting rush between 6pm and 9pm as the sun shone in many parts of Scotland.

An SNP source said: "We are set for a good night, there is no question about that.

"We reckon we have about 30,000 people engaged in getting the vote out today. We are seeing the benefit of being a big membership party."

The source said the SNP expected to benefit from "differential voting" - getting a higher proportion of their known supporters to the polls than other parties.

A Labour source said party organisers were reporting a good response from supporters. The party remained hopeful of salvaging a number of seats, after one poll suggested the SNP might take all 59 seats in Scotland.

A Conservative spokesman said: "People are highly motivated. We are not having to pull people out of their doors, put it that way.

"There seems to have been a lot of tactical voting, particularly in Edinburgh."

The Lib Dems continued to concentrate on the 11 Scottish seats they won in 2010. "A good night for us will be doing better than the pundits believe we will do," a source said.