Mexico on Monday accused Bolivia of intimidating its diplomats in La Paz after a chill in relations since Mexican President Andres Manuel Lopez Obrador last month gave political asylum to former Bolivian president Evo Morales, a fellow leftist.
Bolivia's government responded by saying Mexico had hijacked a regional summit, and had dragged its feet in recognizing its new, conservative premier.
On Tuesday, the Mexican Foreign Ministry said Bolivian police were registering the movements of people coming and going from diplomatic facilities, as well as tracking diplomatic vehicles and trying to prevent the ambassador from moving freely.
"These actions are obviously not in line with usual practices of surveillance and protection of diplomatic buildings ... and can only be explained in the light of the internal political situation," the ministry said in a statement.
Mexico complained of "excessive" surveillance by about 150 Bolivian intelligence and security service personnel on Monday.
Lopez Obrador has declined to comment on what had motivated Bolivia to act as his government alleged. Earlier on Tuesday, he had said that things were improving.
"I have news that this situation of extreme surveillance on our embassy in Bolivia has eased considerably," he told his regular news conference on Tuesday.
Even so, "many things" still needed clearing up, he said.
Bolivia's presidential secretary, Erick Foronda, said security personnel were placed around the Mexican embassy to protect the site because it is housing Morales allies, state news outlet Bolivian Information Agency (ABI) reported.
He added the increase of officers did not affect the free movement of embassy officials.
Bolivia's foreign minister, Karen Longaric, said on Monday that Bolivian authorities would respect diplomatic privilege and not enter Mexico's embassy.
Bolivia has moved to the right since Morales resigned in November after a presidential election that the Organization of American States said was rigged in his favor.
Bolivia is currently governed by interim President Jeanine Añez, a former conservative senator and opponent of Morales.Lopez Obrador said Mexico was eager to know if new elections would be held in Bolivia and whether all "political forces" would be able to take part in them.