YANGON (Reuters) - The party of Myanmar Nobel peace prize laureate Aung San Suu Kyi complained on Monday about attempts to stifle its campaign for April parliamentary by-elections.

The National League for Democracy (NLD), which boycotted the 2010 election, said several attempts to hold campaign rallies had been blocked and accused the country's biggest party of making promises that were tantamount to vote-buying.

"There are increasing restrictions on the election campaign of the NLD, posing threats to the upcoming by-election, making it difficult to be free and fair," Nyan Win, a top NLD official and its campaign manager, told a news conference.

The April 1 by-election vote for 48 vacant seats, mostly in the lower house, will be closely watched by the international community, with a fair contest demanded by Western countries currently reviewing their policies on sanctions in response to democratic reforms by the new civilian government.

Suu Kyi is standing as a candidate in what is seen as a partial endorsement of the fledgling democratic system now in place in Myanmar after decades of authoritarian military rule.

On the campaign trail, supporters have lined the streets and flocked in their thousands to hear speeches by Suu Kyi, the daughter of Myanmar's late independence hero, Aung San.

NEW RESTRICTIONS

Nyan Win accused Sports Minister Tint Hsan of blocking the NLD from holding a rally at a sports ground in the town of Hlegu, north of Yangon, on February 14.

The election commission moved to allow the rally to take place, but that intervention was short-lived, Nyan Win said.

"To our surprise, on February 15, a directive was issued banning campaigns on sports grounds. These incidents really affect the campaign activities of the NLD," he added.

He also questioned a decision by authorities in volatile Kachin State, where conflict is raging between ethnic Kachin rebels and government troops, to reject its request to hold a rally there on security grounds.

"So why do they hold by-elections in a constituency where they cannot ensure security?" Nyan Win said.

Despite Suu Kyi's popularity, the NLD has only limited funds and campaign experience. Suu Kyi is almost certain to win in her constituency, but it is unclear how her party will fare elsewhere.

The NLD must compete with the Union Solidarity and Development Party (USDP), which enjoys big spending power, a parliamentary majority and close connections with the former junta, remnants of which now lead the civilian government.

Suu Kyi's decision to run and her cordial ties with the reform-minded President Thein Sein have been welcomed by the international community, but some critics are skeptical, saying she could be being used by the government to boost its case for having sanctions lifted.

Nyan Win also said some USDP candidates had made promises to voters that infrastructure and electricity upgrades would take place and questioned their source of funding. He warned legal action would be taken if state funds were used.

"Giving such promises is tantamount to buying votes and applying undue influence," Nyan Win said.

(Reporting by Aung Hla Tun; Editing by Martin Petty and Ed Lane)