After meandering in mildly negative territory all morning, sellers stepped up the pace Tuesday afternoon to push stocks lower as volume climbed.

The Nasdaq fell 1.4%, the S&P 500 1.1% and the Dow Jones industrial average 0.7%. Volume climbed 14% on the Nasdaq and 19% on the NYSE, preliminary data showed.

The more aggressive selling may have been sparked by reports of violence in Madrid, where anti-austerity protesters clashed with police. The government will unveil its 2013 budget Thursday, and it will likely include more painful cuts to the public sector. Spain already has slashed outlays for such items as education, health care and public-sector pay.

Two IBD 50 stocks broke out, despite the market’s weakness.

Drug-benefits manager Catamaran (CTRX) climbed 2% in fast trade and cleared a 97.48 buy point from a cup-with-handle base. Group (WWW) had jumped as much as 6% earlier in the day, clearing an 18.40 buy point from a cup-with-handle base. But the stock ended with just a 1% gain, pushing it back below its buy point by day’s end.

Cirrus Logic (CRUS) gave up earlier gains, slid 4% and suffered its fifth loss in a row. But the stock also neared potential support at its 50-day moving average. Cirrus provides audio chips for Apple (AAPL) products.

Apple fell 3% in average trade as the tech sector struggled.

Mellanox Technologies (MLNX) lost 5% and breached its 50-day line. Volume was slightly below average. Mellanox’s 50-day moving average had acted as support recently as the stock bounced back after falling on news earlier this month of the retirement of its chief financial officer. It hadn’t closed below the line in more than two months.

Nationstar Mortgage Holdings (NSM) ended with a 3% high-volume gain, but had been up as shot up 8% before the broad-market sell-off chilled this stock’s hot session.

Earlier, the Case-Shiller 20-City home price index rose 1.2% in July, the sixth straight monthly gain. The mortgage servicer is well extended from a rebound off its 10-week moving average.

On Wednesday, more housing market data is due out, namely the new-home sales report.