Thousands of tickets have been sold for 45-minute guided tours of the Kelpies, which stand 30 metres tall over the Forth & Clyde Canal between Falkirk and Grangemouth.
The £4.95 tours include a unique chance to see inside sculptor Andy Scott's horses' heads, and learn about how they were built, the inspiration behind them and their links to local history and mythology.
Some 300 people were "champing at the bit" to tour the landmarks on the sell-out first day. Hundreds of others arrived just to take a look on their Easter Monday holiday.
The Kelpies, which had a spectacular fire and light-show launch last week, form the centrepiece of the £43 million Helix development which is expected to attract an extra 300,000 people - and £1.5million - per year to the Falkirk area.
Gordon and Lesley Archibald and their children Jenny, six, and nine-year-old Cameron, from Larbert, bought their tickets online as soon as they went on sale, just to be among the first inside. PE teacher Mr Archibald, 39, said: "I've been taking Cameron on drives past the Kelpies for months watching their progress, so we were very keen to be the first to see them from inside too. It was well worth it - there's something special about being the first. They really are fantastic, and I'm hoping they will bring more money and investment to the area."
Cameron said: "I couldn't wait to come because I think the Kelpies are brilliant, and I love the structure inside. It's cool to be one of the first to get to see them like this."
Tommy Brines, 52, and his wife Liz, 54, and children Rudi, four, and Sophie, five, travelled from Paisley to be on the very first guided tour. Mr Brines, who works in heavy engineering, said: "They're amazing, and to go inside was a unique experience. It's a great piece of architecture and their setting on the canal is perfect for them as Kelpies are mythical water horses."
There will be ten tours each day, taking 30 people at a time.