LONDON — Early market relief at the future of the euro zone turned to negativity by midmorning Monday, as stocks and the euro retreated and Spain’s borrowing costs pushed up to around the 7 percent barrier seen by many as unsustainably high.


European and Asian markets opened immediately higher Monday after the Greek legislative election Sunday handed victory to a center-right party, New Democracy, that supports Greece staying in the currency union. That had eased fears that the country will leave the euro and unleash further turmoil on the beleaguered single currency.


But by midmorning Europe had given up those gains, with most major indexes down from Friday’s close.


And although reaction from political and economic leaders was broadly optimistic about the chances for the euro zone, opinions were tempered by the size of the challenges ahead for Greece and for Europe.


The election results are destined to spark a period of intense horsetrading among political parties in Greece to form a coalition government, followed by a second negotiation to secure concessions on loan terms from international lenders.


While that process gets under way, leaders of the Group of 20 nations were due to meet later Monday in Los Cabos, Mexico, with the euro zone crisis likely to figure prominently in discussions.


Comments made there Sunday by the World Bank president, Robert B. Zoellick, may have led markets to penalize Spain, said one European Union official speaking on condition of anonymity due to the sensitivity of the issue.


Mr. Zoellick said European leaders had increased market uncertainty by fumbling the announcement of a €100 billion, or $126 billion, banking rescue for Spain at the beginning of last week. They “took a very big bullet, and wasted it,” Mr. Zoellick said.


But the markets may also be looking at the difficulties ahead in Greece, where coalition talks are expected to begin Monday.


“There will be a roller coaster in the financial markets over the next week or two, with reactions to every announcement about the difficulties of forming a coalition, ” said the E.U. official. “Most people believe that it is not going to be easy in Greece, but we have the least, worst alternative.”


In Athens, the leader of New Democracy, Antonis Samaras, is due to meet the Greek president, Karolos Papoulias, at midday Monday to receive a mandate to form a government.


The Greek vote should give European leaders a brief respite, relieving some of the intense pressure on them ahead of a crucial summit in Brussels at the end of the month, when they are due to discuss fundamental changes to the structure of the single currency.


But the scale of support for the Greek radical leftist party Syriza, which won more than a quarter of the vote, underlined the extent of the opposition in Greece to the strict austerity terms being imposed on the country by international lenders, and suggests that the euro zone is likely to face weeks, or even months, of further political tension.


“Is euphoria appropriate in these circumstances? I wouldn’t say so,” said Kenneth Wattret, chief euro zone economist at BNP Paribas in London. “But there is relief because the election outcome could have been more problematic.”


“The question is how long that lasts,” he said. “We have been in this crisis for some time and markets have got used to something positive happening and then enthusiasm starts to wane.”


In late morning trading in Europe, the EuroStoxx index, which groups European blue chips, was down 0.28 percent. Spanish shares were down 1.56 percent.


In Asian trading, the Nikkei 225 stock average in Tokyo rose 1.77 percent, while the Hang Seng index in Hong Kong rose 1.01 percent.


The euro traded at $1.2647, up slightly from $1.2638 late Friday in New York.


The yield on the Spanish 10-year government bond was up 22 basis points at 7..025 percent. The yield on comparable Italian debt rose 14 basis points to 6.04 percent.