The lower house was due to vote on the bill last Thursday but had to postpone the session because of the disturbances.
The government says the reforms will help tackle the large fiscal deficit, according to a BBC report.
But the protesters accuse the government of balancing the books at the expense of the poor.
Demonstrators threw stones, bottles, rocks and petrol bombs, while police in riot gear responded with tear gas, rubber bullets and water cannon.
The authorities said 48 people had been arrested and at least 100 injured.
The country's main trade union has called a 24-hour general strike and hundreds of airline flights were grounded because of the stoppage.
The conservative President, Mauricio Macri, says the reforms will help tackle the country's deficit, and encourage investment by lowering business costs.
But the bill has angered pensioners and the country's left wing including the powerful trade unions.
Social activists and union leaders complain it would cut pension and retirement payments as well as aid to poor families.
The reforms would change the formula used to calculate benefits and ultimately slow the pace of pension benefit increases.
They are concerned it will take longer to build up pension levels and force people to work for longer, and the pension they would get would be smaller.
A vote on the bill was suspended on Thursday due to similar violent clashes.
The government later amended the pension bill to include a bonus payment for some retirees but this was not enough to stop the protests.
President Macri has drawn the fury of his opponents by vowing to rein in government spending that had expanded hugely during the government of his rival, former president Cristina Fernandez.
He has promised to revive Argentina's flagging economy but his job cuts and the cutting of utility subsidies have fuelled labour unrest.
In October he announced more reforms saying he was seeking changes in tax, education and labour.