BRASILIA, Jan 18 (Reuters) - Brazilian civil servants began a partial strike on Tuesday to demand wage increases, adding pressure on the federal budget in an election year after President Jair Bolsonaro voiced support for giving raises to security forces.

The two-hour stoppage was likely to be just the start of mounting pressure on the cash-strapped government from public employees, many of whom have not seen their wages rise in five years.

Any raises would hit the federal budget, which has posted straight deficits since 2014. Payroll expenses are a huge share of government expenditures, behind only social security costs.

The union of central bank workers protested in front of the institution on Tuesday morning, while about 500 more employees gathered in a virtual assembly.

"We are not going to accept this undignified treatment that the Bolsonaro government has given to public employees," said Rudinei Marques, president of the Fonacate union, which represents 37 categories of civil servants.

Fonacate argues that inflation has already eroded about a quarter of civil servants' salaries, given that 80% have had their salaries frozen since 2017 and the rest since 2019.

A national online protest has been scheduled for Jan. 27, while another demonstration will happen outside the Supreme Court in Brasilia on Feb. 2, Fonacate said.

Marques said the union will decide in the first week of February whether to go on a indefinite strike.

The 2022 budget was approved by Congress with 1.7 billion reais ($308 million) for wage increases, after Bolsonaro said only civil servants providing public security would be allowed to receive raises.

The president has until Friday to sign the budget law.

Bolsonaro's comments set off the mobilization of several other categories of civil servants, who have promised a major strike in February if their wage demands are not met.

Behind the scenes, Economy Minister Paulo Guedes has argued that there should be no increase for any government employees due to the state of the federal budget.

According to a ministry official, there is no easy way to make concessions, as a 1% increase in the payroll means an annual impact of 3 billion reais for the federal government. ($1 = 5.5179 reais) (Reporting by Marcela Ayres; Editing by Marguerita Choy)