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The official result was announced by the Election Commission at about 10.58 pm.
The two-hour delay in the announcement of the result was due to a dispute in the postal ballots where 208 votes were rejected. This however did not change the final result.
According to the EC, the dispute was over the validity of witnesses' signature for postal votes.
Wong, who is also Bukit Assek state assemblyman, garnered 18,845 votes against Barisan National's Robert Lau Hui Yew, who polled 18,447 votes while independent Narawi Haron only received 232 votes.
BN had earlier been expected to retain its Sarawak stronghold but lost despite the best efforts of Prime Minister Datuk Seri Najib Razak.
Pakatan Rakyat has managed to reverse its series of losses (Bagan Pinang and Hulu Selangor) with this win in Sibu.
Of the 11 by-elections since March 2008, Pakatan has won eight while BN three. Sibu is also Pakatan's first victory in East Malaysia. PKR lost in Batang Ai in April last year.
For DAP, this is the first by-election which the party has won in 13 years. Sibu will be the party's second parliamentary seat in Sarawak to its existing Bandar Kuching.
The Sibu by-election was called following the death of five-time MP and Deputy Transport Minister Datuk Robert Lau Hoi Chew on April 9. - reports from the Star and Malaysian Insider.