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Hundreds of dignitaries and senior officials are expected to attend the ceremony at the royal palace in the northern city of Tetouan.
The crowd traditionally awaits the arrival of the king, who appears dressed in white and on horseback before passing through the assembly as those gathered bow and swear loyalty.
The pledge is part of the annual Feast of the Throne ceremony, a nationalist celebration that dates back to the 1930s when Morocco was a French protectorate.
Only official Moroccan media are permitted to cover Wednesday's event, which will also see the swearing-in of new officers.
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The 55-year-old monarch on Tuesday hosted a large reception at his Tangiers palace, where he and Crown Prince Moulay Hassan, 16, were dressed in white traditional robes and red caps.
They greeted scores of guests, while later that evening the king's sisters held a dinner "in honour of women".
On Monday night the king pledged a government reshuffle and an injection of "new blood" into political and administrative positions to help tackle the country's glaring inequalities.
During the speech at his Tetouan palace, he welcomed progress in infrastructure and freedoms in the country but said the efforts had not had "sufficient impact".
Morocco's unemployment rate hovers around 10 percent, but spikes up to 39 percent among those under 24, according to official figures.
Access to quality healthcare remains problematic in rural areas and education levels are low, with one in three Moroccans illiterate, official figures say.
As is tradition, the king's speech on Monday was followed by a royal pardoning, which this year was handed to thousands of prisoners, including some from the "Hirak" protest movement that rocked the country in 2016.
Of the 4,764 included in the pardon, only eight were detained during the months of protest in the long-marginalised northern Rif region, and none of them were leaders of the movement.
The Al-Hirak al-Shaabi, or "Popular Movement", was sparked by the death of a fisherman but soon spiralled into demands for more development and action against corruption and unemployment.
More than 400 protesters are thought to have been arrested and tried in connection with the demonstrations, but no official figures are available. Around 250 of them have previously been pardoned.