He referred to the AfD's result as "depressing," saying that widespread fears about the refugee crisis had fuelled support for the right-wing party.
"This is a turning point," said Schulz. "It's clear that the decision to welcome refugees has divided our society. What is a great act of humanity to some seems threatening to others. We didn't manage to convince people that Germany is strong enough to leave no one behind."
Lisa Price, official of SPD's state commission in Brandenburg, told Xinhua that refusing the grand coalition (CDU/CSU plus SPD) is SPD's victory, because "SPD will finally have the opportunity to prove itself."
"Staying out the grand coalition will make SPD more distinctive and could focus on such issues as the interests of the low-income groups as an opposition party," she told Xinhua.
Gerhard Stahl, a Berliner who has been an SPD since 1970s, now professor with College of Europe in Brussels, said Schulz should not hold the whole responsibility for SPD's loss.
"Being an opposition party can also give the people an alternative. A lot of people are dissatisfied with the ruling coalition, if the two largest party group form a grand coalition again, there will be no alternative for voters. If the government let us down in the coming years, we can vote for SPD instead," Stahl told Xinhua.
"The result (of SPD) is foreseeable. It is a difficult situation with immigrants this time around, but you should not overlook that there are 85 percent voters who had voted normal parliamentary parties, it showed the strength of German system that all these parties can cooperate and Germany is a stable democracy," Stahl told Xinhua.
AFD'S CELEBRATIONS MET WITH PROTEST
In another development, the AfD crossed the five-percent-vote threshold on Sunday came into the new parliament for the first time. It is also the first far-right populist party in the Bundestag since WWII.
"We will hunt Merkel! We want to bring our country and people back," said Alexander Gauland, one of the two candidates of the AfD in this election who always accuses Merkel's policies in refugees and euro crisis.
"It's a historical and outstanding result for AfD. We will experience more pluralism in the Bundestag, and we will experience a lively democracy through the AfD," said AfD's politician Bjoern Hoecke.
However, Hoecke's joyful words met with protests and demonstrations against AfD shortly after.
Hundreds of demonstrators gathered on Berlin's Alexanderplatz on Sunday evening, bearing umbrellas to keep dry and waving anti-AfD signs to protest AfD's election result.
Later, the crowd shouted as one, "The whole of Berlin hates the AfD."
Police were monitoring the situation and prohibiting the protesters from drawing close to the building housing the AfD's party.
According to security authority, AfD supporters were cleared off the building's balcony after demonstrators on the ground threw objects in that direction.
Similar anti-AfD protests also took place on the other side of the country in Cologne. Demonstrators met in front of the west German city's central train station before marching through the streets with a banner reading "Whoever is silent, is complicit".
The two rallies were parts of a nationwide campaign dubbed "Nationalism is not an Alternative", which also saw 300 people rallying in Frankfurt, German news agency DPA reported.
Despite AfD's breakthrough, experts believed that the party's influence should not be overestimated.
Prof. Dr. Paul Nolte, a historian with Free University Berlin, told Xinhua that as SPD refused to form another grand coalition government with the CDU/CSU bloc, the AfD will not become the largest opposition party, so that their influence in the Bundestag will be restricted.
Prof. Dr. Hajo Funke, also a political scientist with Free University Berlin focusing on right-wing extremism, told Xinhua that although the AfD is popular to a small degree in the public, but most others have realized that it is a dangerous right-wing party.
Although it has entered the Bundestag, none of the parties in the federal parliament, no matter the small parties or the country's two major parties, ruled out cooperation with the AfD in any form, possibly making it a lonely marginal man in the Bundestag.
Funke also doubted that the AfD would be stronger in the future, as the party has lost the flexibility.
"Right-wing parties like the FPOE in Austria, they say something very radical but act more like centrists. But when it comes to AfD, its leading structure has decided to be right-wing radical, which has limited their influence," said Funke.
Founded in 2013, the eurosceptic and anti-immigration party -- which welcomed Brexit, missed the Bundestag with 4.7 percent of votes in the same year.
The party gained momentum during the Euro and the refugee crisis. It was polling at around 10 percent before Sunday's federal election.
About 73,500 polling stations across the country opened at 8:00 a.m. local time (0600 GMT) and closed at 6:00 p.m. (1600 GMT).
Official statistics showed that Sunday's turnout of the election stood at 75 percent, higher than the 71.5 percent in 2013.
