* Stoppages confirmed at top grain ports Santos, Paranagua
* Unions to strike again Tuesday, threaten longer walk-out
* Workers protesting government drive to privatize ports
SANTOS, Brazil, Feb 22 (Reuters) - Brazilian dock workers walked off the job in at least two major ports early on Friday for a six-hour strike to protest the government's plans to change regulations and put more of the country's ports in the private sector's hands.
The strike comes in the midst of a record soybean harvest. Expected delays at Brazil's ports caused top buyer China this week to cancel at least two soy cargoes ordered from Brazil and buy from the United States instead.
Brazilian dock workers are afraid a drive to privatize some 158 terminals will lead to a loss of jobs and benefits because private operators would not have to hire through the centralized agency "OGMO" and might bring in labor from abroad.
"We've started the mobilization," said Cesar Rodrigues Alves, a senior representative of the union of stevedores at Santos port, South America's largest.
A press official at the other major grains port, Paranagua, confirmed the labor stoppage had begun there, too.
Shipping analysts do not expect major disruption, given the short duration of the strike. Bulk grain loading, which is done via conveyor belt and requires little labor, is also likely to continue uninterrupted at Santos, shipping agent SA Commodities/Unimar told Reuters.
The Brazilian government says the planned changes for ports are needed to boost competitiveness as it seeks to attract billions of dollars in private investment to expand capacity to cope with burgeoning commodity exports.
Queues of ships waiting to load grains this week were twice as long as this time last year at Santos and three times longer at Paranagua, according to data from SA Commodities/Unimar, with strong demand for an expected record soy crop.
Faced with the prospect of more protests at a time when logistics infrastructure is already under major strain, the country's port minister was scheduled to meet with union representatives to discuss the proposals Friday morning.
Friday's strike at ports across the country will last until 1 p.m. (1600 GMT) on Friday. Umbrella union Forca Sindical said workers would hold another six-hour walkout next Tuesday and were evaluating a longer strike. (Reporting by Gustavo Bonato; Editing by Peter Murphy and Lisa Von Ahn)