Spain has banned short-selling of shares to try to limit price moves after markets fell sharply on fears the country may need a full bailout.
Spain's market regulator blocked the practice for three months to try to restore order after sharp falls in bonds and shares.
"Short-selling" is a way that traders can make money by betting on falling share prices.
Italy has also banned short-selling of financial stocks for one week.
Short-selling is a technique used by investors who think the price of an asset, such as shares, will fall.
They borrow the asset from another investor and then sell it in the relevant market. The aim is to buy back the asset at a lower price and return it to its owner, making a profit along the way.
In a statement, Spain's CNMV regulator said it was imposing the ban in order to maintain market order: "The situation of extreme volatility across the European markets could interfere with their smooth functioning and the normal course of their activities."
It is not the first time that such a curb has been used by regulators. Almost a year ago, France and Belgium joined Spain and Italy in a ban on short-selling financial stocks to try to stabilise bank shares which had fallen sharply.
Markets have had a turbulent few days on fears that Spain's indebted regional governments will push the country towards a full bailout.
On Friday, Valencia, one of the country's 17 regions, asked the central government for a financial lifeline, and on Sunday, the Murcia region said it was considering following suit.
Shares in Europe fell when trading got underway on Monday, with Spain's main share index, the Ibex, down 5% at one point. It recovered slightly to close down 1% but Germany's Dax ended the day down 3%.
The US share markets opened with a downward jolt and the euro hit a new two-year low against the dollar.
Spain's economy minister denied the country needed more help.
Luis de Guindos said: "We have made important economic reforms and we just reached an agreement with our regional partners over the recapitalisation of the banks, and from there we have done all what we could to establish the bases of a return to a healthy growth for Spain's economy."
Mr Guindos is due to meet his German counterpart in Berlin on Tuesday.
Markets remained unsettled. The yield on Spain's 10-year bonds reached a new euro-era high of 7.56% before falling back to 7.39% in late afternoon trading.
The bond yield indicates the interest rate the government would have to pay to borrow new money, and acts as a measure of investor confidence in Spain's creditworthiness.
Spain has already asked for and been granted a 100bn-euros bailout for its banks, so far avoiding asking for the same sort of national bailout that was needed by Greece, the Republic of Ireland and Portugal.
However, on Friday the Valencia region said it would be the first region to seek financial help from an 18bn-euro fund set up to help the country's regions.
On Sunday, Murcia's government said: "Regarding the liquidity fund provided by the state, the regional government has repeatedly stated that it is studying whether to apply for it."
There is speculation that other regions are also considering seeking assistance, creating further pressure on central government finances.
There was more bad news for Spain on Monday when the Bank of Spain said the country's economy contracted by 0.4% in the three months to the end of June, having shrunk by 0.3% in the previous quarter.
Eurozone jitters also spread to Italy, which is also struggling with high debts. The main Italian share index closed down 2.7% with banks being the worst hit. UniCredit and Intesa Sanpaolo were among six Italian banks suspended from trading after their share prices fell sharply.
On the currency markets, the euro fell to a two-year low against the US dollar, at $1.2082 at one point on Monday and an 11-year low against the Japanese yen, 94.37 yen, its lowest level since November 2000, before recovering slightly.
The price of oil has also fallen by nearly 3%, a sign that markets think there will be waning demand for oil as a result of worsening economic prospects.