The Queen attended a service of commemoration in Crathie Kirk in Crathie, Aberdeenshire, near Balmoral.
At 11pm on August 4, 1914, Britain declared war on Germany, ushering in four years of darkness, despair and appalling tragedy.
In the UK to mark its beginning, the Prince of Wales attended a service at Glasgow Cathedral, which was followed by a wreath-laying service and marchpast at the Cenotaph in George Square.
In London at 10pm – an hour before war was officially declared 100 years ago – a service of solemn commemoration got under way at Westminster Abbey.
The service included the gradual extinguishing of candles, with an oil lamp extinguished at the tomb of the unknown soldier at 11pm – the exact hour war was declared. In the same hour, the nation was urged to switch off lights in places of worship, public buildings, workplaces and homes, and leave one light burning as a symbol of hope in darkness, in a reference to then-foreign secretary Sir Edward Grey's remark on the eve of the outbreak of war that the "lamps are going out all over Europe". In Afghanistan, personnel from all three services were joined by the US Marine Corps at Camp Bastion to mark the occasion.
Around 400 personnel congregated at the base's Vigil Site for a parade illuminated by the lights of military vehicles and the site itself.