In Yangon, the commercial capital, authorities appeared to have thwarted a planned ceremony at the famed Shwedagon Pagoda that would have kicked off the boycott there.
"We could not hold the formal ceremony because the Shwedagon was closed," one monk, who declined to be named, told Reuters.
The more than 400 monks then marched peacefully to the city centre, chanting prayers and holy scriptures but no political slogans.
They were trailed by plainclothes police and members of the Union Solidarity and Development Association (USDA), the junta's feared social organisation which has broken up protests against soaring fuel prices that began four weeks ago.

They videotaped and photographed the monks, but took no action against them, witnesses said.
Similar marches were reported in Bago, 50 miles (80 km) north of Yangon, and in the northwestern city of Sittwe where one resident said the monks had been "suppressed" but gave no details.
REFUSING ALMS
The Myanmar-language services of foreign broadcasters have reported that some Buddhist monks would refuse to accept alms from anybody associated with the ruling generals from Tuesday.
They said an alliance of monks had demanded an apology for soldiers firing shots to disperse a demonstration by monks in the town of Pakokku two weeks ago.
A boycott, in which monks refuse to accept alms and offerings from well-wishers, would be taken extremely seriously in the deeply devout country.
Without such rites, a Buddhist loses all chance of attaining nirvana, or release from the cycle of rebirth.
The nationwide boycott being called for looked unlikely, analysts said, although sporadic outbursts of defiance were expected, especially in the provinces.
Although the army has run Myanmar since a 1962 coup, Sept. 18 is the anniversary of the latest incarnation of the junta, which now goes by the name of State Peace and Development Council.
Monks launched a similar boycott in 1990 shortly after the generals refused to honour the results of a general election they lost by a landslide.
The monasteries were key players in a nationwide uprising against military rule in 1988 and analysts say the generals are at pains to treat them carefully this time around.
Official newspapers have given prominent coverage to men in uniform making donations in temples -- and having them accepted -- especially in Mandalay, the nation's second city which is home to 300,000 monks.