The firing on Friday came even as the Nepalese army urged citizens to ignore the blockade call and get on with normal life, promising to protect them from Maoist reprisals.
Two rebels opened fire on police and soldiers guarding Kathmandu's Land Registration office shortly after the building was badly damaged by bombs, said deputy superintendent of police K C Ganesh.
One police officer was injured in the attack and taken to hospital, he said. There was no immediate word on his condition.
The attack was the first major violent incident since the Maoists announced their blockade, and the Nepalese army tried to reassure jittery residents.
"We want to tell vehicle owners to be free from this slavery mentality of terror created by rebel threats," said army spokesman Rajendra Bahadur Thapa. "We want to assure them that we have enough security to protect them."
"We want to tell vehicle owners to be free from this slavery mentality of terror created by rebel threats"
Rajendra Bahadur Thapa,
While the Maoists have put up no roadblocks to stop vehicles entering and leaving Kathmandu, fears about reprisals have kept many trucks, heavy vehicles and cars off major highways since the Maoists announced the blockade.
Thapa said the flow of vehicles coming through the so-called chicken neck leg of highway linking the rest of the country with the Kathmandu valley was increasing, but it was still far below normal.
Maoist guerillas, who are seeking to install a communist republic, have said they will keep up the siege until their demands are met.
The rebels' demands include release of jailed colleagues and the government to stop labelling them as terrorists.