Hungary's election did not offer opposition parties a level playing field amid a host of problems marring a vote that nonetheless generally respected fundamental rights, the Organisation for Security and Cooperation in Europe said on Monday.
"The government's excessive spending on ads that amplified the ruling coalition's campaign message undermined contestants' ability to compete on an equal basis," OSCE election monitor Douglas Wake told a news conference.
"Intimidating and xenophobic rhetoric, media bias and opaque campaign financing" hindered political debate, he said.
"Fundamental rights and freedoms were respected overall, but exercised in an adverse climate ... Access to information as well as the freedoms of the media and association have been restricted, including by recent legal changes."
Right-wing nationalist Viktor Orban swept to victory in the country's general election, with his Fidesz party winning a two-thirds majority for the third straight time - meaning he again has the powers to change constitutional laws.
Orban projected himself as a saviour of Hungary's Christian culture against Muslim migration into Europe, an image which resonated with more than 2.5 million voters, especially in rural areas.
The OSCE's criticism came shortly after Fidesz signalled it could push on quickly with legislation to crack down on organisations promoting migrant rights as soon as parliament reconvenes.
The victory could embolden Orban to put more muscle into a Central European alliance against EU migration policies, working with other right-wing nationalists in Poland and Austria, and further expose cracks in the 28-nation bloc.
A Fidesz spokesman told state radio on Monday: "After parliament is formed, at the end of April ... in early May in the next parliament session we can start work ... that is needed in the interest of the country, which could be the Stop Soros legal package."
The proposed legislation is part of Orban's campaign targeting Hungarian born US financier George Soros, whose philanthropy aims to bolster liberal and open-border values.
Among the measures floated before the election were mandatory registration of some non-governmental organisations that "support illegal immigration" and a 25 percent tax to be imposed on foreign donations that such NGOs collect, as well as restraining orders that preclude activists from approaching the EU's external borders in Hungary.
Those borders have been fortified since a migrant influx in 2015, when hundreds of thousands of people fled wars and poverty in the Middle East and Africa.
One non-governmental organisation described the prospect of the bill as "terrifyingly serious".
Last month, Orban told state radio the government had drafted the bill because activists were being paid by Soros to "transform Hungary into an immigrant country".
Soros has rejected the government campaign against him as "distortions and lies" meant to create a false external enemy.
Analysts at HSBC said Orban's strong mandate "could be problematic if it serves to embolden its nationalistic policies and strengthens its hand in its arguments with the EU over the rule of law or migration".
A spokesman said European Commission President Jean-Claude Juncker would write to Orban to congratulate him on his victory.
"The people of Hungary have voted yesterday and the European Union is a union of democracy and values," the European Commission spokesman added.
"President Juncker and the Commission feel that defending these principles and defending these values is the common duty of all member states with no exception."
German Chancellor Angela Merkel congratulated Orban, a German government spokesman said, adding she would work with his new government despite differences on migration.
"It is quite obvious that there are also controversial issues in our cooperation, the different stances in migration policy come to mind," spokesman Steffen Seibert added.
French far-right leader Marine Le Pen was the first to congratulate Orban. Poland's deputy foreign minister and envoy to the EU, Konrad Szymanski, hailed his victory as "a confirmation of Central Europe's emancipation policy".
According to preliminary results with 99 percent of votes counted, National Election Office data showed Fidesz winning 133 seats, a tight two-thirds majority in the 199-seat parliament. Nationalist Jobbik won 26 seats, while the Socialists were projected in third with 20 lawmakers.
Some of the NGOs that could be hit by the new law said they expected a hardening in the new government's stance.
"With a two-thirds majority, there can be no doubt they can and will do it," Hungarian Civil Liberties Union director Stefania Kapronczay said. "This is terrifyingly serious."
Transparency International Legal Director Miklos Ligeti said for the time being it considered Halasz' comment "a political declaration made in the heat of an election victory".
Orban, Hungary's longest-serving post-communist premier, opposes deeper integration of the EU and - teaming up with Poland - has been a fierce critique of Brussels' policies.
Since coming to power in 2010, his government has locked horns with the European Commission over reforms that critics say have eroded democratic checks and balances and weakened the independence of the media.
His critics say Orban has put Hungary on an increasingly authoritarian path and his stance on immigration has fuelled xenophobia in the Central European country.
In the capital Budapest, where Fidesz won only six out of 18 voting districts, some were disappointed on Monday morning.
"Well, the government has successfully implemented its hate campaign. They planted hatred in people's heart, which is very sad," said Balazs Bansagi, 45, a quality controller.