By Tommy Stubbington and Josie Cox 

Greek stocks and bonds on Monday felt the weight of the success of antiausterity party Syriza in the country's election, but the impact on European markets more broadly was mild.

Athens' main stock index initially slumped by nearly 5%, and was down 3.2% at the close. Bank shares were sharply lower, with Piraeus Bank SA tumbling more than 17% to the bottom of the pan-European index. Peers Eurobank Ergasias SA and Alpha Bank AE both ended more than 10% lower.

The victory for Syriza could set Athens on a collision cause with its creditors and embolden radical parties elsewhere to challenge Europe's economic orthodoxy. Analysts and investors said they had anticipated a victory for Syriza given the party's lead in the polls, but the size of the margin was something of a surprise.

Maria Paola Toschi, a market strategist at J.P. Morgan Asset Management, which looks after around $1.7 trillion globally, said that banks in Greece are "the biggest potential losers from the extended period of uncertainty that markets may be in for."

Greek government bonds were also under pressure, with 10-year yields climbing to 8.91%, reflecting a fall in prices. Shorter-dated debt declined even more sharply, with two-year yields rising more than 1.5 percentage points to around 11.5%. Investors demanding higher yields to hold short-term bonds compared with those on longer-term debt is often a sign buyers are factoring in a significant chance of default.

But weakness in the euro--which has already chalked up big losses this year--was short-lived, with the currency picking up against the dollar and the yen in European trade. Traders and analysts said some investors were cashing in on their negative euro bets after recent declines.

Wider equity markets in Europe were unruffled by the news, with stocks closing in on recent seven-year highs after a small early wobble. The Stoxx Europe 600 index ended 0.6% higher. In the U.S., the S&P 500 was 0.2% higher at the European close.

Investors appear confident that the bond-buying stimulus program known as quantitative easing, announced last week by the European Central Bank leaves markets much less vulnerable to fears of a eurozone breakup than they were during previous episodes of the Greek crisis.

"Political risk is back on the radar in Europe, but markets are still under the influence of the ECB's liquidity injection," said François Savary, who oversees about $10 billion of assets as chief investment officer at Swiss bank Reyl.

"We expect that [ECB] backstops have effectively firewalled Greek developments and should limit contagion or re-emergence of [eurozone] existential anxiety. As such we don't expect developments to translate into substantial further euro weakness beyond what is already justified by QE," said currency strategists at BNP Paribas.

Stephen Thariyan, global head of credit at Henderson Global Investors, which manages GBP79.9 billion ($120 billion), echoed this.

"As an investor I predict normality and stability, given that in my opinion Greece wants to stay [in the eurozone]," he said.

"The path to that scenario will not be without its moments, but they are moments, which I believe Europe can deal with -- Europe has a central bank that has bite. As for this vote, I am inclined to buy on weakness.," he added.

In Asian trading hours, the euro dipped to a fresh 11-year low to the dollar of just below $1.11. Ultrasafe German debt rallied to new all-time highs while bonds in fiscally weak eurozone countries declined.

The result gives Syriza "a strong mandate" to push through major changes to the Greek adjustment program, said Jan von Gerich, chief strategist at Nordea.

"The ensuing negotiations will be tough and contribute to market volatility in the coming months," he added.

Syriza's decision to strike a deal with the small Independent Greeks party could lead to more strained negotiations with Greece's creditors--given the two parties share little except a rejection of the austerity measures imposed on Greece by its creditors.

Wider pressure on bond markets in the eurozone was modest on Monday.

Italian and Spanish 10-year yields rose slightly to 1.54% and 1.39%. Still, both countries' yields quickly fell back and remain within touching distance of the all-time lows hit in the wake of last Thursday's ECB meeting.

German yields, which typically fall in times of stress, touched a record low of below 0.3% on the 10-year bond before rising to trade higher on the day at 0.36%.

Write to Tommy Stubbington at tommy.stubbington@wsj.com and Josie Cox at josie.cox@wsj.com

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