The Maoists and the government last month signed a peace deal declaring an end to a decade-old anti-monarchy rebellion that has killed more than 13,000 people.
The latest strike action was not expected by the government.
"I am really surprised," KP Sharma Oli, Nepal's deputy prime minister and foreign minister, said in reaction to the protest.
The ambassadors had been recalled by the interim administration, which was set up after King Gyanendra ceded power following mass protests in April.
The peace deal was signed after weeks of dispute, mainly concerning the draft of an interim constitution and management of the rebels' arms.
The roadmap to a final political settlement in Nepal includes elections to a special assembly next year which would write a new constitution and decide the fate of Nepal's monarchy.