France's upper house of parliament began on Tuesday the examination of the controversial pension reforms, while labor unions pledged further massive protests to push President Nicolas Sarkozy's government to bow to their calls.

According to labor unions' figures, about 4,000 protesters gathered in front of the Senate House in an attempt to force the senators not to vote on the controversial pension bill. They waved flags of CGT, the country's main trade union, and chanted slogans like "Retirement at 60, we fought to win, we will fight to keep" and "In this society there, we do not want it."

The French Interior Ministry said only 2,000 people participated in the demonstration.

"We hope that the French's participation will be more important to support unions, which assume a great responsibility and are united to fight," Martine Aubry, the head of the Socialist party who participated in the demonstration, told the state-run TV channel France 2.

The Senate is likely to pass the bill, which has been adopted by the lower house of parliament.

The upper house debate on pension reforms is expected to last until Oct. 15.

The opposition parties, which enjoy a majority in the Senate, proposed more than 1,000 amendments and a motion to submit a referendum draft on the first day of the bill's examination.

"The government would not reconsider raising age limits ... We must respect the general balance of the text ... If public pensions are no longer insured, middle-class people will be more penalized," Labor Minister Eric Woerth told the senators while presenting the bill.

The unions have planned another nationwide strike next Tuesday in order to make their voices heard and press the officials to give up the reforms.

However, the government has so far stood firm on pension reforms, which would raise the French people's retirement age from 60 to 62 by 2018, to create financial resources enough to help the country narrow its budget gap.