The strike has prompted warnings of "severe disruption" across Edinburgh and the Lothians.
More than 10,000 drivers and mail centre staff across Scotland walked out at midnight after hopes of reaching a last-minute deal between unions and Royal Mail failed yesterday evening.
Today's action will involve around 1,200 workers across the Lothians, who are protesting over long-running disputes about Royal Mail's modernisation plans, pay and working conditions
Speaking from the picket line outside the Mail Centre in Cultins Road early this morning, CWU area processing representative Graham Steedman said: "There's about 20 members here and that will increase as the morning goes on and people start turning up. We're quite upbeat. Obviously the last thing we want is to be out on strike and causing disruption, but we were left with no choice."
Further action is planned for tomorrow, to involve 2,000 postal delivery and collection staff in the local area.
A spokeswoman for the Royal Mail said: "Unfortunately, it is likely there will be severe disruption to delivery services on Friday. The strikes may cause delays. We will do everything we can to keep these to an absolute minimum."
Customers were urged this morning to bring any mail directly and promptly to post offices for postage, or to put it in postboxes directly outside branches to ensure the best chance of delivery.
They were also advised to use the Royal Mail's Special Delivery service for any urgent mail.
She said: "We'll continue to prioritise the delivery of Special Delivery and Royal Mail tracked items, but can't guarantee these services on strike days."
An estimated 120,000 postal workers across the UK will be involved in the two-day strikes which are expected to cripple the service.
The Communication Workers Union has warned of further strikes in the coming weeks, having launched an attack last night on Business Secretary Lord Mandelson, insisting he was working "hand-in-hand" with the Royal Mail to "undermine" the dispute.
Royal Mail managing director Mark Higson wrote to the union yesterday to urge members to abandon the "totally unjustified" strike.
But the union's deputy general secretary Dave Ward insisted the Royal Mail had no intention of resolving the dispute and pushed forward with strike action, stressing there was no alternative.
A spokesman for CWU in the Lothians said: "It is unfortunate it's had to come to a dispute like this, for the disruption it will cause to customers and small businesses."
Courier firms across the Lothians have reported a small rise in business following the announcement of strike action as customers opt to pay a higher price to guarantee delivery on time.
Mark Elliot, a partner with City Couriers on Beaufort Road, said: "We usually see an increase when anything like this happens and Wednesday was particularly busy, especially for overnight national deliveries. It's a case of people not trusting the Royal Mail."
Royal Mail stressed Post Office and Parcelforce staff are unaffected by the strike action and are working as normal.